<s>
The	O
Linux	B-Application
kernel	B-Operating_System
is	O
a	O
free	B-License
and	I-License
open-source	I-License
,	O
monolithic	B-Operating_System
,	O
modular	B-Architecture
,	O
multitasking	B-Operating_System
,	O
Unix-like	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
kernel	B-Operating_System
.	O
</s>
<s>
It	O
was	O
originally	O
authored	O
in	O
1991	O
by	O
Linus	O
Torvalds	O
for	O
his	O
i386-based	O
PC	O
,	O
and	O
it	O
was	O
soon	O
adopted	O
as	O
the	O
kernel	B-Operating_System
for	O
the	B-Operating_System
GNU	I-Operating_System
operating	I-Operating_System
system	I-Operating_System
,	O
which	O
was	O
written	O
to	O
be	O
a	O
free	O
(	O
libre	O
)	O
replacement	O
for	O
Unix	B-Application
.	O
</s>
<s>
Linux	B-Application
is	O
provided	O
under	O
the	O
GNU	B-License
General	I-License
Public	I-License
License	I-License
version	I-License
2	I-License
only	I-License
,	O
but	O
it	O
contains	O
files	O
under	O
other	O
compatible	O
licenses	O
.	O
</s>
<s>
Since	O
the	O
late	O
1990s	O
,	O
it	O
has	O
been	O
included	O
as	O
part	O
of	O
a	O
large	O
number	O
of	O
operating	B-Application
system	I-Application
distributions	I-Application
,	O
many	O
of	O
which	O
are	O
commonly	O
also	O
called	O
Linux	B-Application
.	O
</s>
<s>
Linux	B-Application
is	O
deployed	O
on	O
a	O
wide	O
variety	O
of	O
computing	O
systems	O
,	O
such	O
as	O
embedded	B-Application
devices	I-Application
,	O
mobile	B-Application
devices	I-Application
(	O
including	O
its	O
use	O
in	O
the	O
Android	B-Application
operating	I-Application
system	I-Application
)	O
,	O
personal	B-Device
computers	I-Device
,	O
servers	B-Application
,	O
mainframes	B-Architecture
,	O
and	O
supercomputers	B-Architecture
.	O
</s>
<s>
It	O
can	O
be	O
tailored	O
for	O
specific	O
architectures	B-General_Concept
and	O
for	O
several	O
usage	O
scenarios	O
using	O
a	O
family	O
of	O
simple	O
commands	O
(	O
that	O
is	O
,	O
without	O
the	O
need	O
of	O
manually	O
editing	O
its	O
source	O
code	O
before	O
compilation	B-Language
)	O
;	O
privileged	O
users	O
can	O
also	O
fine-tune	O
kernel	B-Operating_System
parameters	O
at	B-Operating_System
runtime	O
.	O
</s>
<s>
Most	O
of	O
the	O
Linux	B-Application
kernel	B-Operating_System
code	O
is	O
written	O
using	O
the	O
GNU	B-Operating_System
extensions	O
of	O
GCC	B-Application
to	O
the	O
standard	O
C	B-Language
programming	I-Language
language	I-Language
and	O
with	O
the	O
use	O
of	O
architecture-specific	O
instructions	O
(	O
ISA	B-General_Concept
)	O
in	O
limited	O
parts	O
of	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
This	O
produces	O
a	O
highly	O
optimized	O
executable	O
(	O
vmlinux	B-Application
)	O
with	O
respect	O
to	O
utilization	O
of	O
memory	O
space	O
and	O
task	O
execution	O
times	O
.	O
</s>
<s>
Day-to-day	O
development	O
discussions	O
take	O
place	O
on	O
the	O
Linux	B-Application
kernel	I-Application
mailing	I-Application
list	I-Application
(	O
LKML	B-Application
)	O
.	O
</s>
<s>
Changes	O
are	O
tracked	O
using	O
the	O
version	B-Architecture
control	I-Architecture
system	I-Architecture
git	B-Application
,	O
which	O
was	O
originally	O
authored	O
by	O
Torvalds	O
as	O
a	O
free	B-Application
software	I-Application
replacement	O
for	O
BitKeeper	B-Protocol
.	O
</s>
<s>
In	O
April	O
1991	O
,	O
Linus	O
Torvalds	O
,	O
at	B-Operating_System
the	O
time	O
a	O
21-year-old	O
computer	B-General_Concept
science	I-General_Concept
student	O
at	B-Operating_System
the	O
University	O
of	O
Helsinki	O
,	O
Finland	O
,	O
started	O
working	O
on	O
some	O
simple	O
ideas	O
for	O
an	O
operating	O
system	O
inspired	O
by	O
UNIX	B-Application
,	O
for	O
a	O
personal	B-Device
computer	I-Device
.	O
</s>
<s>
He	O
started	O
with	O
a	O
task	B-Operating_System
switcher	I-Operating_System
in	O
Intel	B-Language
80386	I-Language
assembly	I-Language
language	I-Language
and	O
a	O
terminal	B-Protocol
driver	I-Protocol
.	O
</s>
<s>
On	O
25	O
August	O
1991	O
,	O
Torvalds	O
posted	O
the	O
following	O
to	O
comp.os.minix	O
,	O
a	O
newsgroup	B-Application
on	O
Usenet	B-Application
:	O
</s>
<s>
On	O
17	O
September	O
1991	O
,	O
Torvalds	O
prepared	O
version	O
0.01	O
of	O
Linux	B-Application
and	O
put	O
on	O
the	O
"	O
ftp.funet.fi	O
 "	O
–	O
FTP	O
server	O
of	O
the	O
Finnish	O
University	O
and	O
Research	O
Network	O
(	O
FUNET	O
)	O
.	O
</s>
<s>
It	O
was	O
not	O
even	O
executable	O
since	O
its	O
code	O
still	O
needed	O
Minix	B-Operating_System
to	O
compile	B-Language
and	O
to	O
test	O
it	O
.	O
</s>
<s>
On	O
5	O
October	O
1991	O
,	O
Torvalds	O
announced	O
the	O
first	O
"	O
official	O
"	O
version	O
of	O
Linux	B-Application
,	O
version	O
0.02	O
.	O
</s>
<s>
At	B-Operating_System
this	O
point	O
,	O
Linux	B-Application
was	O
able	O
to	O
run	O
Bash	B-Operating_System
,	O
GCC	B-Application
,	O
and	O
some	O
other	O
GNU	B-Operating_System
utilities	B-Application
:	O
</s>
<s>
After	O
that	O
,	O
despite	O
the	O
limited	O
functionality	O
of	O
the	O
early	O
versions	O
,	O
Linux	B-Application
rapidly	O
gained	O
developers	O
and	O
users	O
.	O
</s>
<s>
Many	O
people	O
contributed	O
code	O
to	O
the	O
project	O
,	O
including	O
some	O
developers	O
from	O
the	O
MINIX	B-Operating_System
community	O
.	O
</s>
<s>
At	B-Operating_System
the	O
time	O
,	O
the	B-Application
GNU	I-Application
Project	I-Application
had	O
created	O
many	O
of	O
the	O
components	O
required	O
for	O
its	O
free	O
UNIX	B-Application
replacement	O
,	O
the	B-Operating_System
GNU	I-Operating_System
operating	I-Operating_System
system	I-Operating_System
,	O
but	O
its	O
own	O
kernel	B-Operating_System
,	O
GNU	B-Application
Hurd	I-Application
,	O
was	O
incomplete	O
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
it	O
soon	O
adopted	O
the	O
Linux	B-Application
kernel	B-Operating_System
as	O
well	O
.	O
</s>
<s>
The	O
Berkeley	B-Operating_System
Software	I-Operating_System
Distribution	I-Operating_System
had	O
not	O
yet	O
freed	O
itself	O
from	O
legal	O
encumbrances	O
and	O
was	O
not	O
competing	O
in	O
the	O
space	O
for	O
a	O
free	O
OS	B-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
Torvalds	O
assigned	O
version	O
0	O
to	O
the	O
kernel	B-Operating_System
to	O
indicate	O
that	O
it	O
was	O
mainly	O
for	O
testing	O
and	O
not	O
intended	O
for	O
productive	O
use	O
.	O
</s>
<s>
Version	O
0.11	O
,	O
released	O
in	O
December	O
1991	O
,	O
was	O
the	O
first	O
self-hosted	O
Linux	B-Application
,	O
for	O
it	O
could	O
be	O
compiled	B-Language
by	O
a	O
computer	O
running	O
the	O
same	O
kernel	B-Operating_System
.	O
</s>
<s>
When	O
Torvalds	O
released	O
version	O
0.12	O
in	O
February	O
1992	O
,	O
he	O
adopted	O
the	O
GNU	B-License
General	I-License
Public	I-License
License	I-License
version	O
2	O
(	O
GPLv2	O
)	O
over	O
his	O
previous	O
self-drafted	O
license	O
,	O
which	O
had	O
not	O
permitted	O
commercial	O
redistribution	O
.	O
</s>
<s>
In	O
contrast	O
to	O
Unix	B-Application
,	O
all	O
source	O
files	O
of	O
Linux	B-Application
are	O
freely	O
available	O
,	O
including	O
device	B-Application
drivers	I-Application
.	O
</s>
<s>
The	O
initial	O
success	O
of	O
Linux	B-Application
was	O
driven	O
by	O
programmers	O
and	O
testers	O
across	O
the	O
world	O
.	O
</s>
<s>
With	O
the	O
support	O
of	O
the	O
POSIX	O
APIs	B-Application
,	O
through	O
the	O
libC	B-Language
that	O
,	O
whether	O
needed	O
,	O
acts	O
as	O
an	O
entry	O
point	O
to	O
the	O
kernel	B-Operating_System
address	O
space	O
,	O
Linux	B-Application
could	O
run	O
software	O
and	O
applications	O
that	O
had	O
been	O
developed	O
for	O
Unix	B-Application
.	O
</s>
<s>
On	O
19	O
January	O
1992	O
,	O
the	O
first	O
post	O
to	O
the	O
new	O
newsgroup	B-Application
alt.os.linux	O
was	O
submitted	O
.	O
</s>
<s>
On	O
31	O
March	O
1992	O
,	O
the	O
newsgroup	B-Application
was	O
renamed	O
comp.os.linux	O
.	O
</s>
<s>
The	O
fact	O
that	O
Linux	B-Application
is	O
a	O
monolithic	B-Operating_System
kernel	I-Operating_System
rather	O
than	O
a	O
microkernel	B-Operating_System
was	O
the	O
topic	O
of	O
a	O
debate	O
between	O
Andrew	O
S	O
.	O
Tanenbaum	O
,	O
the	O
creator	O
of	O
MINIX	B-Operating_System
,	O
and	O
Torvalds	O
.	O
</s>
<s>
The	O
Tanenbaum	B-Language
–	I-Language
Torvalds	I-Language
debate	I-Language
started	O
in	O
1992	O
on	O
the	O
Usenet	B-Application
group	I-Application
comp.os.minix	O
as	O
a	O
general	O
discussion	O
about	O
kernel	B-Operating_System
architectures	B-General_Concept
.	O
</s>
<s>
Linux	B-Application
version	O
0.95	O
was	O
the	O
first	O
to	O
be	O
capable	O
of	O
running	O
the	O
X	B-Operating_System
Window	I-Operating_System
System	I-Operating_System
.	O
</s>
<s>
In	O
March	O
1994	O
,	O
Linux	B-Application
1.0.0	O
was	O
released	O
with	O
176,250	O
lines	O
of	O
code	O
.	O
</s>
<s>
It	O
started	O
a	O
versioning	B-Architecture
system	I-Architecture
for	O
the	O
kernel	B-Operating_System
with	O
three	O
or	O
four	O
numbers	O
separated	O
by	O
dots	O
where	O
the	O
first	O
represented	O
the	O
major	O
release	O
,	O
the	O
second	O
was	O
the	O
minor	O
release	O
,	O
and	O
the	O
third	O
was	O
the	O
revision	O
.	O
</s>
<s>
At	B-Operating_System
that	O
time	O
odd-numbered	O
minor	O
releases	O
were	O
for	O
development	O
and	O
tests	O
,	O
whilst	O
even	O
numbered	O
minor	O
releases	O
were	O
for	O
production	O
.	O
</s>
<s>
After	O
version	O
1.3	O
of	O
the	O
kernel	B-Operating_System
,	O
Torvalds	O
decided	O
that	O
Linux	B-Application
had	O
evolved	O
enough	O
to	O
warrant	O
a	O
new	O
major	O
number	O
,	O
so	O
he	O
released	O
version	O
2.0.0	O
in	O
June	O
1996	O
.	O
</s>
<s>
The	O
major	O
feature	O
of	O
2.0	O
was	O
support	O
for	O
symmetric	B-Operating_System
multiprocessing	I-Operating_System
(	O
SMP	B-Operating_System
)	O
and	O
support	O
for	O
more	O
types	O
of	O
processors	O
.	O
</s>
<s>
Starting	O
with	O
version	O
2.0	O
,	O
Linux	B-Application
is	O
configurable	O
for	O
selecting	O
specific	O
hardware	O
targets	O
and	O
for	O
enabling	O
architecture-specific	O
features	O
and	O
optimizations	O
.	O
</s>
<s>
The	O
make	B-Application
*	O
config	O
family	O
of	O
commands	O
of	O
kbuild	O
are	O
used	O
to	O
enable	O
and	O
configure	O
thousands	O
of	O
options	O
for	O
building	O
ad	O
hoc	O
kernel	B-Operating_System
executables	O
(	O
vmlinux	B-Application
)	O
and	O
loadable	O
modules	B-Application
.	O
</s>
<s>
Version	O
2.2	O
,	O
released	O
on	O
20	O
January	O
1999	O
,	O
improved	O
locking	O
granularity	O
and	O
SMP	B-Operating_System
management	O
,	O
added	O
m68k	B-Device
,	O
PowerPC	B-Architecture
,	O
Sparc64	B-Architecture
,	O
Alpha	B-Device
,	O
and	O
other	O
64-bit	B-Device
platforms	O
support	O
.	O
</s>
<s>
Furthermore	O
,	O
it	O
added	O
new	O
file	B-Application
systems	I-Application
including	O
Microsoft	O
's	O
NTFS	B-Application
read-only	O
capability	O
.	O
</s>
<s>
In	O
1999	O
,	O
IBM	O
published	O
its	O
patches	O
to	O
the	O
Linux	B-Application
2.2.13	O
code	O
for	O
the	O
support	O
of	O
the	O
S/390	B-Device
architecture	O
.	O
</s>
<s>
Version	O
2.4.0	O
,	O
released	O
on	O
4	O
January	O
2001	O
,	O
contained	O
support	O
for	O
ISA	B-General_Concept
Plug	B-Device
and	I-Device
Play	I-Device
,	O
USB	B-Protocol
,	O
and	O
PC	B-Architecture
Cards	I-Architecture
.	O
</s>
<s>
Linux	B-Application
2.4	O
added	O
support	O
for	O
the	O
Pentium	B-General_Concept
4	I-General_Concept
and	O
Itanium	B-General_Concept
(	O
the	O
latter	O
introduced	O
the	O
ia64	B-General_Concept
ISA	B-General_Concept
that	O
was	O
jointly	O
developed	O
by	O
Intel	O
and	O
Hewlett-Packard	O
to	O
supersede	O
the	O
older	O
PA-RISC	B-Device
)	O
,	O
and	O
for	O
the	O
newer	O
64-bit	B-Device
MIPS	I-Device
processor	O
.	O
</s>
<s>
Development	O
for	O
2.4.x	O
changed	O
a	O
bit	O
in	O
that	O
more	O
features	O
were	O
made	O
available	O
throughout	O
the	O
duration	O
of	O
the	O
series	O
,	O
including	O
support	O
for	O
Bluetooth	B-Protocol
,	O
Logical	B-Application
Volume	I-Application
Manager	I-Application
(	O
LVM	O
)	O
version	O
1	O
,	O
RAID	B-Architecture
support	O
,	O
InterMezzo	B-Application
and	O
ext3	B-Application
file	B-Application
systems	I-Application
.	O
</s>
<s>
The	O
development	O
for	O
2.6.x	O
changed	O
further	O
towards	O
including	O
new	O
features	O
throughout	O
the	O
duration	O
of	O
the	O
series	O
.	O
</s>
<s>
Among	O
the	O
changes	O
that	O
have	O
been	O
made	O
in	O
the	O
2.6	O
series	O
are	O
:	O
integration	O
of	O
µClinux	O
into	O
the	O
mainline	O
kernel	B-Operating_System
sources	O
,	O
PAE	B-General_Concept
support	O
,	O
support	O
for	O
several	O
new	O
lines	O
of	O
CPUs	B-Device
,	O
integration	O
of	O
Advanced	B-Application
Linux	I-Application
Sound	I-Application
Architecture	I-Application
(	O
ALSA	B-Application
)	O
into	O
the	O
mainline	O
kernel	B-Operating_System
sources	O
,	O
support	O
for	O
up	O
to	O
232	O
users	O
(	O
up	O
from	O
216	O
)	O
,	O
support	O
for	O
up	O
to	O
229	O
process	O
IDs	O
(	O
64-bit	B-Device
only	O
,	O
32-bit	O
arches	O
still	O
limited	O
to	O
215	O
)	O
,	O
substantially	O
increased	O
the	O
number	O
of	O
device	O
types	O
and	O
the	O
number	O
of	O
devices	O
of	O
each	O
type	O
,	O
improved	O
64-bit	B-Device
support	I-Device
,	O
support	O
for	O
file	B-Application
systems	I-Application
which	O
support	O
file	O
sizes	O
of	O
up	O
to	O
16	O
terabytes	O
,	O
in-kernel	O
preemption	B-Operating_System
,	O
support	O
for	O
the	O
Native	B-Application
POSIX	I-Application
Thread	I-Application
Library	I-Application
(	O
NPTL	B-Application
)	O
,	O
User-mode	B-Application
Linux	I-Application
integration	O
into	O
the	O
mainline	O
kernel	B-Operating_System
sources	O
,	O
SELinux	B-Application
integration	O
into	O
the	O
mainline	O
kernel	B-Operating_System
sources	O
,	O
InfiniBand	B-Architecture
support	O
,	O
and	O
considerably	O
more	O
.	O
</s>
<s>
Also	O
notable	O
are	O
the	O
addition	O
of	O
a	O
wide	O
selection	O
of	O
file	B-Application
systems	I-Application
starting	O
with	O
the	O
2.6.x	O
releases	O
:	O
now	O
,	O
the	O
kernel	B-Operating_System
supports	O
a	O
large	O
number	O
of	O
file	B-Application
systems	I-Application
,	O
some	O
that	O
have	O
been	O
designed	O
for	O
Linux	B-Application
,	O
like	O
ext3	B-Application
,	O
ext4	B-Application
,	O
FUSE	B-Application
,	O
Btrfs	B-Operating_System
,	O
and	O
others	O
that	O
are	O
native	O
of	O
other	O
operating	O
systems	O
like	O
JFS	B-Application
,	O
XFS	B-Application
,	O
Minix	B-Operating_System
,	O
Xenix	B-Application
,	O
Irix	B-Operating_System
,	O
Solaris	B-Application
,	O
System	B-Operating_System
V	I-Operating_System
,	O
Windows	B-Application
and	O
MS-DOS	B-Application
.	O
</s>
<s>
In	O
2005	O
the	O
stable	O
team	O
was	O
formed	O
as	O
a	O
response	O
to	O
the	O
lack	O
of	O
a	O
kernel	B-Operating_System
tree	O
where	O
people	O
could	O
work	O
on	O
bug	B-Error_Name
fixes	I-Error_Name
,	O
and	O
it	O
would	O
keep	O
updating	O
stable	O
versions	O
.	O
</s>
<s>
In	O
February	O
2008	O
the	O
linux-next	O
tree	O
was	O
created	O
to	O
serve	O
as	O
a	O
place	O
where	O
patches	O
aimed	O
to	O
be	O
merged	O
during	O
the	O
next	O
development	O
cycle	O
gathered	O
.	O
</s>
<s>
,	O
the	O
in-development	O
version	O
of	O
Linux	B-Application
is	O
held	O
in	O
an	O
unstable	O
branch	O
named	O
linux-next	O
.	O
</s>
<s>
Linux	B-Application
used	O
to	O
be	O
maintained	O
without	O
the	O
help	O
of	O
an	O
automated	O
source	B-Architecture
code	I-Architecture
management	I-Architecture
system	O
until	O
,	O
in	O
2002	O
,	O
development	O
switched	O
to	O
BitKeeper	B-Protocol
.	O
</s>
<s>
It	O
was	O
freely	O
available	O
for	O
Linux	B-Application
developers	O
but	O
it	O
was	O
not	O
free	B-Application
software	I-Application
.	O
</s>
<s>
In	O
2005	O
,	O
because	O
of	O
efforts	O
to	O
reverse-engineer	O
it	O
,	O
the	O
company	O
which	O
owned	O
the	O
software	O
revoked	O
its	O
support	O
of	O
the	O
Linux	B-Application
community	I-Application
.	O
</s>
<s>
In	O
response	O
,	O
Torvalds	O
and	O
others	O
wrote	O
Git	B-Application
.	O
</s>
<s>
The	O
new	O
system	O
was	O
written	O
within	O
weeks	O
,	O
and	O
in	O
two	O
months	O
the	O
first	O
official	O
kernel	B-Operating_System
made	O
using	O
it	O
was	O
released	O
.	O
</s>
<s>
Details	O
on	O
the	O
history	O
of	O
the	O
2.6	O
kernel	B-Operating_System
series	O
can	O
be	O
found	O
in	O
the	O
ChangeLog	O
files	O
on	O
the	O
2.6	O
kernel	B-Operating_System
series	O
source	O
code	O
release	O
area	O
of	O
kernel.org	B-Application
.	O
</s>
<s>
The	O
20th	O
anniversary	O
of	O
Linux	B-Application
was	O
celebrated	O
by	O
Torvalds	O
in	O
July	O
2011	O
with	O
the	O
release	O
of	O
the	O
3.0.0	O
kernel	B-Operating_System
version	O
.	O
</s>
<s>
As	O
2.6	O
had	O
been	O
the	O
version	O
number	O
for	O
8	O
years	O
,	O
a	O
new	O
uname26	O
personality	O
that	O
reports	O
3.x	O
as	O
2.6.40	O
+x	O
had	O
to	O
be	O
added	O
to	O
the	O
kernel	B-Operating_System
so	O
that	O
old	O
programs	O
would	O
work	O
.	O
</s>
<s>
and	O
asked	O
,	O
"	O
let	O
...	O
'	O
s	O
make	B-Application
sure	O
we	O
really	O
make	B-Application
the	O
next	O
release	O
not	O
just	O
an	O
all	O
new	O
shiny	O
number	O
,	O
but	O
a	O
good	O
kernel	B-Operating_System
too.	O
"	O
</s>
<s>
After	O
the	O
expected	O
6	O
–	O
7	O
weeks	O
of	O
the	O
development	O
process	O
,	O
it	O
would	O
be	O
released	O
near	O
the	O
20th	O
anniversary	O
of	O
Linux	B-Application
.	O
</s>
<s>
On	O
11	O
December	O
2012	O
,	O
Torvalds	O
decided	O
to	O
reduce	O
kernel	B-Operating_System
complexity	O
by	O
removing	O
support	O
for	O
i386	B-General_Concept
processors	O
,	O
making	O
the	O
3.7	O
kernel	B-Operating_System
series	O
the	O
last	O
one	O
still	O
supporting	O
the	O
original	O
processor	O
.	O
</s>
<s>
The	O
same	O
series	O
unified	O
support	O
for	O
the	O
ARM	B-Architecture
processor	I-Architecture
.	O
</s>
<s>
Version	O
3.11	O
,	O
released	O
on	O
2	O
September	O
2013	O
,	O
adds	O
many	O
new	O
features	O
such	O
as	O
new	O
flag	O
for	O
to	O
reduce	O
temporary	O
file	O
vulnerabilities	O
,	O
experimental	O
AMD	B-Device
Radeon	I-Device
dynamic	O
power	O
management	O
,	O
low-latency	O
network	O
polling	O
,	O
and	O
zswap	B-Application
(	O
compressed	O
swap	O
cache	O
)	O
.	O
</s>
<s>
Stable	O
3.x.y	O
kernels	B-Operating_System
were	O
released	O
until	O
3.19	O
in	O
February	O
2015	O
.	O
</s>
<s>
In	O
April	O
2015	O
,	O
Torvalds	O
released	O
kernel	B-Operating_System
version	O
4.0	O
.	O
</s>
<s>
By	O
February	O
2015	O
,	O
Linux	B-Application
had	O
received	O
contributions	O
from	O
nearly	O
12,000	O
programmers	O
from	O
more	O
than	O
1,200	O
companies	O
,	O
including	O
some	O
of	O
the	O
world	O
's	O
largest	O
software	O
and	O
hardware	O
vendors	O
.	O
</s>
<s>
Version	O
4.1	O
of	O
Linux	B-Application
,	O
released	O
in	O
June	O
2015	O
,	O
contains	O
over	O
19.5	O
million	O
lines	O
of	O
code	O
contributed	O
by	O
almost	O
14,000	O
programmers	O
.	O
</s>
<s>
According	O
to	O
the	O
Stack	O
Overflow	O
's	O
annual	O
Developer	O
Survey	O
of	O
2019	O
,	O
more	O
than	O
the	O
53%	O
of	O
all	O
respondents	O
have	O
developed	O
software	O
for	O
Linux	B-Application
and	O
about	O
27%	O
for	O
Android	B-Application
,	O
although	O
only	O
about	O
25%	O
develop	O
with	O
Linux-based	B-Application
operating	I-Application
systems	I-Application
.	O
</s>
<s>
Most	O
websites	O
run	O
on	O
Linux-based	B-Application
operating	I-Application
systems	I-Application
,	O
and	O
all	O
of	O
the	O
world	B-Operating_System
's	I-Operating_System
500	I-Operating_System
most	I-Operating_System
powerful	I-Operating_System
supercomputers	I-Operating_System
use	O
some	O
kind	O
of	O
OS	O
based	O
on	O
Linux	B-Application
.	O
</s>
<s>
Linux	B-Application
distributions	I-Application
bundle	O
the	O
kernel	B-Operating_System
with	O
system	B-Application
software	I-Application
(	O
e.g.	O
,	O
the	O
GNU	B-Language
C	I-Language
Library	I-Language
,	O
systemd	B-Application
,	O
and	O
others	O
Unix	B-Application
utilities	B-Application
and	O
daemons	B-Operating_System
)	O
and	O
a	O
wide	O
selection	O
of	O
application	B-Application
software	I-Application
,	O
but	O
their	O
usage	O
share	O
in	O
desktops	O
is	O
low	O
in	O
comparison	O
to	O
other	O
operating	O
systems	O
.	O
</s>
<s>
Android	B-Application
,	O
which	O
accounts	O
for	O
the	O
majority	O
of	O
the	O
installed	O
base	O
of	O
all	O
operating	O
systems	O
for	O
mobile	B-Application
devices	I-Application
,	O
is	O
responsible	O
for	O
the	O
rising	O
usage	O
of	O
the	O
Linux	B-Application
kernel	B-Operating_System
,	O
together	O
with	O
its	O
wide	O
use	O
in	O
a	O
large	O
variety	O
of	O
embedded	B-Application
devices	I-Application
.	O
</s>
<s>
Linux	B-Application
is	O
a	O
monolithic	B-Operating_System
kernel	I-Operating_System
with	O
a	O
modular	B-Architecture
design	O
(	O
i.e.	O
,	O
it	O
can	O
insert	O
and	O
remove	O
loadable	B-Application
kernel	I-Application
modules	I-Application
at	B-Operating_System
runtime	O
)	O
,	O
supporting	O
most	O
features	O
once	O
only	O
available	O
in	O
closed	B-Application
source	I-Application
kernels	B-Operating_System
of	O
non-free	O
operating	O
systems	O
.	O
</s>
<s>
The	O
rest	O
of	O
the	O
article	O
makes	O
use	O
of	O
the	O
UNIX	B-Application
and	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
convention	O
on	O
the	O
official	O
manual	B-Protocol
pages	I-Protocol
.	O
</s>
<s>
The	O
numbers	O
that	O
follow	O
the	O
name	O
of	O
commands	O
,	O
interfaces	O
,	O
and	O
other	O
features	O
,	O
have	O
the	O
purpose	O
of	O
specifying	O
the	O
section	O
(	O
i.e.	O
,	O
the	O
type	O
of	O
the	O
OS	O
 '	O
component	O
or	O
feature	O
)	O
they	O
belong	O
to	O
(	O
e.g.	O
,	O
refers	O
to	O
a	O
system	B-Operating_System
call	I-Operating_System
,	O
while	O
refers	O
to	O
a	O
userspace	B-Operating_System
library	O
wrapper	O
)	O
.	O
</s>
<s>
The	O
following	O
list	O
and	O
the	O
subsequent	O
sections	O
describe	O
a	O
non-comprehensive	O
overview	O
of	O
Linux	B-Application
architectural	O
design	O
and	O
of	O
some	O
of	O
its	O
noteworthy	O
features	O
.	O
</s>
<s>
Concurrent	B-Architecture
computing	I-Architecture
and	O
(	O
with	O
the	O
availability	O
of	O
enough	O
CPU	B-Device
cores	O
for	O
tasks	O
that	O
are	O
ready	O
to	O
run	O
)	O
even	O
true	B-Operating_System
parallel	I-Operating_System
execution	I-Operating_System
of	O
many	O
processes	B-Operating_System
at	B-Operating_System
once	O
(	O
each	O
of	O
them	O
having	O
one	O
or	O
more	O
threads	B-Operating_System
of	I-Operating_System
execution	I-Operating_System
)	O
on	O
SMP	B-Operating_System
and	O
NUMA	B-Operating_System
architectures	B-General_Concept
.	O
</s>
<s>
Selection	O
and	O
configuration	O
of	O
hundreds	O
of	O
kernel	B-Operating_System
features	O
and	O
drivers	O
(	O
using	O
one	O
of	O
the	O
family	O
of	O
commands	O
,	O
before	O
running	O
compilation	B-Language
)	O
,	O
modification	O
of	O
kernel	B-Operating_System
parameters	O
before	O
booting	B-Application
(	O
usually	O
by	O
inserting	O
instructions	O
into	O
the	O
lines	O
of	O
the	O
GRUB2	O
menu	O
)	O
,	O
and	O
fine	O
tuning	O
of	O
kernel	B-Operating_System
behavior	O
at	B-Operating_System
run-time	O
(	O
using	O
the	O
interface	O
to	O
)	O
.	O
</s>
<s>
Configuration	O
(	O
again	O
using	O
the	O
commands	O
)	O
and	O
run-time	O
modifications	O
of	O
the	O
policies	O
(	O
via	O
,	O
,	O
and	O
the	O
family	O
of	O
syscalls	B-Operating_System
)	O
of	O
the	O
task	O
schedulers	O
that	O
allow	O
preemptive	O
multitasking	B-Operating_System
(	O
both	O
in	O
user	B-Operating_System
mode	I-Operating_System
and	O
,	O
since	O
the	O
2.6	O
series	O
,	O
in	O
kernel	B-Operating_System
mode	O
)	O
;	O
the	O
Completely	B-Application
Fair	I-Application
Scheduler	I-Application
(	O
CFS	B-Application
)	O
is	O
the	O
default	O
scheduler	O
of	O
Linux	B-Application
since	O
2007	O
and	O
it	O
uses	O
a	O
red-black	O
tree	O
which	O
can	O
search	O
,	O
insert	O
and	O
delete	O
process	O
information	O
(	O
task	B-Operating_System
struct	I-Operating_System
)	O
with	O
O(log n )	O
time	O
complexity	O
,	O
where	O
n	O
is	O
the	O
number	O
of	O
runnable	O
tasks	O
.	O
</s>
<s>
Advanced	O
memory	B-General_Concept
management	I-General_Concept
with	O
paged	B-Architecture
virtual	I-Architecture
memory	I-Architecture
.	O
</s>
<s>
Inter-process	B-Operating_System
communications	I-Operating_System
and	O
synchronization	O
mechanism	O
.	O
</s>
<s>
A	O
virtual	B-Application
filesystem	I-Application
on	O
top	O
of	O
several	O
concrete	O
filesystems	B-Application
(	O
ext4	B-Application
,	O
Btrfs	B-Operating_System
,	O
XFS	B-Application
,	O
JFS	B-Application
,	O
FAT32	B-Application
,	O
and	O
many	O
more	O
)	O
.	O
</s>
<s>
Configurable	O
I/O	O
schedulers	O
,	O
syscall	B-Operating_System
that	O
manipulates	O
the	O
underlying	O
device	O
parameters	O
of	O
special	B-Application
files	I-Application
(	O
it	O
is	O
a	O
non	O
standard	O
system	B-Operating_System
call	I-Operating_System
,	O
since	O
arguments	O
,	O
returns	O
,	O
and	O
semantics	O
depends	O
on	O
the	O
device	B-Application
driver	I-Application
in	O
question	O
)	O
,	O
support	O
for	O
POSIX	O
asynchronous	O
I/O	O
(	O
however	O
,	O
because	O
they	O
scale	O
poorly	O
with	O
multithreaded	O
applications	O
,	O
a	O
family	O
of	O
Linux	B-Application
specific	O
I/O	O
system	B-Operating_System
calls	I-Operating_System
(	O
)	O
had	O
to	O
be	O
created	O
for	O
the	O
management	O
of	O
asynchronous	O
I/O	O
contexts	O
suitable	O
for	O
concurrently	O
processing	O
)	O
.	O
</s>
<s>
OS-level	B-Application
virtualization	I-Application
(	O
with	O
Linux-VServer	B-Application
)	O
,	O
paravirtualization	O
and	O
hardware-assisted	B-General_Concept
virtualization	I-General_Concept
(	O
with	O
KVM	B-Application
or	O
Xen	B-Operating_System
,	O
and	O
using	O
QEMU	B-Application
for	O
hardware	O
emulation	O
)	O
;	O
On	O
the	O
Xen	B-Operating_System
hypervisor	I-Operating_System
,	O
the	O
Linux	B-Application
kernel	B-Operating_System
provides	O
support	O
to	O
build	O
Linux	B-Application
distributions	I-Application
(	O
such	O
as	O
openSuSE	O
Leap	O
and	O
many	O
others	O
)	O
that	O
work	O
as	O
Dom0	B-Operating_System
,	O
that	O
are	O
virtual	O
machine	O
host	O
servers	B-Application
that	O
provide	O
the	O
management	O
environment	O
for	O
the	O
user	O
's	O
virtual	O
machines	O
(	O
DomU	B-Operating_System
)	O
.	O
</s>
<s>
I/O	B-Protocol
Virtualization	I-Protocol
with	O
VFIO	B-Protocol
and	O
SR-IOV	B-Protocol
.	O
</s>
<s>
Virtual	O
Function	O
I/O	O
(	O
VFIO	B-Protocol
)	O
exposes	O
direct	O
device	O
access	O
to	O
user	B-Operating_System
space	I-Operating_System
in	O
a	O
secure	B-General_Concept
memory	I-General_Concept
(	O
IOMMU	O
)	O
protected	O
environment	O
.	O
</s>
<s>
With	O
VFIO	B-Protocol
,	O
a	O
VM	O
Guest	O
can	O
directly	O
access	O
hardware	O
devices	O
on	O
the	O
VM	O
Host	O
Server	O
.	O
</s>
<s>
However	O
,	O
with	O
VFIO	B-Protocol
,	O
devices	O
cannot	O
be	O
shared	O
with	O
multiple	O
VM	O
guests	O
.	O
</s>
<s>
Single	O
Root	O
I/O	B-Protocol
Virtualization	I-Protocol
(	O
SR-IOV	B-Protocol
)	O
combines	O
the	O
performance	O
gains	O
of	O
VFIO	B-Protocol
and	O
the	O
ability	O
to	O
share	O
a	O
device	O
with	O
several	O
VM	O
Guests	O
(	O
but	O
it	O
requires	O
special	O
hardware	O
that	O
must	O
be	O
capable	O
to	O
appear	O
to	O
two	O
or	O
more	O
VM	O
guests	O
as	O
different	O
devices	O
)	O
.	O
</s>
<s>
Security	O
mechanisms	O
for	O
discretionary	O
and	O
mandatory	O
access	O
control	O
(	O
SELinux	B-Application
,	O
AppArmor	O
,	O
POSIX	O
ACLs	O
,	O
and	O
others	O
)	O
.	O
</s>
<s>
Several	O
types	O
of	O
layered	O
communication	O
protocols	O
(	O
including	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
)	O
.	O
</s>
<s>
Asymmetric	B-Operating_System
multiprocessing	I-Operating_System
via	O
the	O
RPMsg	B-Protocol
subsystem	O
.	O
</s>
<s>
Most	O
device	B-Application
drivers	I-Application
and	O
kernel	B-Application
extensions	I-Application
run	O
in	O
kernel	B-Operating_System
space	I-Operating_System
(	O
ring	B-Operating_System
0	I-Operating_System
in	O
many	O
CPU	B-General_Concept
architectures	I-General_Concept
)	O
,	O
with	O
full	O
access	O
to	O
the	O
hardware	O
.	O
</s>
<s>
Some	O
exceptions	O
run	O
in	O
user	B-Operating_System
space	I-Operating_System
;	O
notable	O
examples	O
are	O
filesystems	B-Application
based	O
on	O
FUSE/CUSE	O
,	O
and	O
parts	O
of	O
UIO	O
.	O
</s>
<s>
Furthermore	O
,	O
the	O
X	B-Operating_System
Window	I-Operating_System
System	I-Operating_System
and	O
Wayland	B-Language
,	O
the	O
windowing	O
system	O
and	O
display	O
server	O
protocols	O
that	O
most	O
people	O
use	O
with	O
Linux	B-Application
,	O
do	O
not	O
run	O
within	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
Differently	O
,	O
the	O
actual	O
interfacing	O
with	O
GPUs	B-Architecture
of	O
graphics	B-Device
cards	I-Device
is	O
an	O
in-kernel	O
subsystem	O
called	O
Direct	B-Application
Rendering	I-Application
Manager	I-Application
(	O
DRM	B-Application
)	O
.	O
</s>
<s>
Unlike	O
standard	O
monolithic	B-Operating_System
kernels	I-Operating_System
,	O
device	B-Application
drivers	I-Application
are	O
easily	O
configured	O
as	O
modules	B-Application
,	O
and	O
loaded	O
or	O
unloaded	O
while	O
the	O
system	O
is	O
running	O
and	O
can	O
also	O
be	O
pre-empted	O
under	O
certain	O
conditions	O
in	O
order	O
to	O
handle	O
hardware	O
interrupts	B-Application
correctly	O
and	O
to	O
better	O
support	O
symmetric	B-Operating_System
multiprocessing	I-Operating_System
.	O
</s>
<s>
By	O
choice	O
,	O
Linux	B-Application
has	O
no	O
stable	O
device	B-Application
driver	I-Application
application	B-Operating_System
binary	I-Operating_System
interface	I-Operating_System
.	O
</s>
<s>
Linux	B-Application
typically	O
makes	O
use	O
of	O
memory	B-General_Concept
protection	I-General_Concept
and	O
virtual	B-Architecture
memory	I-Architecture
and	O
can	O
also	O
handle	O
non-uniform	B-Operating_System
memory	I-Operating_System
access	I-Operating_System
,	O
however	O
the	O
project	O
has	O
absorbed	O
μClinux	B-Application
which	O
also	O
makes	O
it	O
possible	O
to	O
run	O
Linux	B-Application
on	O
microcontrollers	B-Architecture
without	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
User	O
applications	O
interact	O
with	O
device	B-Application
drivers	I-Application
via	O
entries	O
in	O
the	O
or	O
directories	O
.	O
</s>
<s>
Processes	B-Operating_System
information	O
as	O
well	O
are	O
mapped	O
to	O
the	O
file	B-Application
system	I-Application
through	O
the	O
directory	O
.	O
</s>
<s>
Linux	B-Application
is	O
a	O
clone	O
of	O
UNIX	B-Application
,	O
and	O
aims	O
toward	O
POSIX	O
and	O
Single	O
UNIX	B-Application
Specification	O
compliance	O
.	O
</s>
<s>
The	O
kernel	B-Operating_System
also	O
provides	O
system	B-Operating_System
calls	I-Operating_System
and	O
other	O
interfaces	O
that	O
are	O
Linux-specific	O
.	O
</s>
<s>
In	O
order	O
to	O
be	O
included	O
in	O
the	O
official	O
kernel	B-Operating_System
,	O
the	O
code	O
must	O
comply	O
with	O
a	O
set	O
of	O
licensing	O
rules	O
.	O
</s>
<s>
The	O
Linux	B-Application
Application	B-Operating_System
binary	I-Operating_System
interface	I-Operating_System
(	O
ABI	B-Operating_System
)	O
between	O
the	O
kernel	B-Operating_System
and	O
the	O
user	B-Operating_System
space	I-Operating_System
has	O
four	O
degrees	O
of	O
stability	O
(	O
stable	O
,	O
testing	O
,	O
obsolete	O
,	O
removed	O
)	O
;	O
however	O
,	O
the	O
system	B-Operating_System
calls	I-Operating_System
are	O
expected	O
to	O
never	O
change	O
in	O
order	O
to	O
not	O
break	O
the	O
userspace	B-Operating_System
programs	O
that	O
rely	O
on	O
them	O
.	O
</s>
<s>
Loadable	B-Application
kernel	I-Application
modules	I-Application
(	O
LKMs	O
)	O
,	O
by	O
design	O
,	O
cannot	O
rely	O
on	O
a	O
stable	O
ABI	B-Operating_System
.	O
</s>
<s>
Therefore	O
,	O
they	O
must	O
always	O
be	O
recompiled	B-Language
whenever	O
a	O
new	O
kernel	B-Operating_System
executable	O
is	O
installed	O
in	O
a	O
system	O
,	O
otherwise	O
they	O
will	O
not	O
be	O
loaded	O
.	O
</s>
<s>
In-tree	O
drivers	O
that	O
are	O
configured	O
to	O
become	O
an	O
integral	O
part	O
of	O
the	O
kernel	B-Operating_System
executable	O
(	O
vmlinux	B-Application
)	O
are	O
statically	O
linked	O
by	O
the	O
building	O
process	O
.	O
</s>
<s>
There	O
is	O
also	O
no	O
guarantee	O
of	O
stability	O
of	O
source-level	O
in-kernel	O
API	B-Application
and	O
,	O
because	O
of	O
this	O
,	O
device	B-Application
drivers	I-Application
code	O
,	O
as	O
well	O
as	O
the	O
code	O
of	O
any	O
other	O
kernel	B-Operating_System
subsystem	O
,	O
must	O
be	O
kept	O
updated	O
with	O
kernel	B-Operating_System
evolution	O
.	O
</s>
<s>
Any	O
developer	O
who	O
makes	O
an	O
API	B-Application
change	O
is	O
required	O
to	O
fix	O
any	O
code	O
that	O
breaks	O
as	O
the	O
result	O
of	O
their	O
change	O
.	O
</s>
<s>
The	O
set	O
of	O
the	O
Linux	B-Application
kernel	B-Operating_System
API	B-Application
that	O
regards	O
the	O
interfaces	O
exposed	O
to	O
user	O
applications	O
is	O
fundamentally	O
composed	O
of	O
UNIX	B-Application
and	O
Linux-specific	O
system	B-Operating_System
calls	I-Operating_System
.	O
</s>
<s>
A	O
system	B-Operating_System
call	I-Operating_System
is	O
an	O
entry	O
point	O
into	O
the	O
Linux	B-Application
kernel	B-Operating_System
.	O
</s>
<s>
For	O
example	O
,	O
among	O
the	O
Linux-specific	O
ones	O
there	O
is	O
the	O
family	O
of	O
the	O
system	B-Operating_System
calls	I-Operating_System
.	O
</s>
<s>
Most	O
extensions	O
must	O
be	O
enabled	O
by	O
defining	O
the	O
_GNU_SOURCE	O
macro	O
in	O
a	O
header	B-Language
file	I-Language
or	O
when	O
the	O
user-land	O
code	O
is	O
being	O
compiled	B-Language
.	O
</s>
<s>
System	B-Operating_System
calls	I-Operating_System
can	O
only	O
be	O
invoked	O
by	O
using	O
assembly	B-Language
instructions	O
which	O
enable	O
the	O
transition	O
from	O
unprivileged	O
user	B-Operating_System
space	I-Operating_System
to	O
privileged	O
kernel	B-Operating_System
space	I-Operating_System
in	O
ring	B-Operating_System
0	I-Operating_System
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
the	B-Language
C	I-Language
standard	I-Language
library	I-Language
(	O
libC	B-Language
)	O
acts	O
as	O
a	O
wrapper	O
to	O
most	O
Linux	B-Application
system	B-Operating_System
calls	I-Operating_System
,	O
by	O
exposing	O
C	B-Language
functions	I-Language
that	O
,	O
only	O
whether	O
it	O
is	O
needed	O
,	O
can	O
transparently	O
enter	O
into	O
the	O
kernel	B-Operating_System
which	O
will	O
execute	O
on	O
behalf	O
of	O
the	O
calling	O
process	O
.	O
</s>
<s>
For	O
those	O
system	B-Operating_System
calls	I-Operating_System
not	O
exposed	O
by	O
libC	B-Language
,	O
e.g.	O
</s>
<s>
the	O
fast	O
userspace	B-Operating_System
mutex	B-Operating_System
(	O
futex	B-Application
)	O
,	O
the	O
library	O
provides	O
a	O
function	O
called	O
which	O
can	O
be	O
used	O
to	O
explicitly	O
invoke	O
them	O
.	O
</s>
<s>
Pseudo	B-Application
filesystems	I-Application
(	O
e.g.	O
,	O
the	O
sysfs	B-Application
and	O
procfs	B-Application
filesystems	B-Application
)	O
and	O
special	B-Application
files	I-Application
(	O
e.g.	O
,	O
/dev/random	O
,	O
/dev/sda	O
,	O
/dev/tty	O
,	O
and	O
many	O
others	O
)	O
constitute	O
another	O
layer	O
of	O
interface	O
to	O
kernel	B-Operating_System
data	O
structures	O
representing	O
hardware	O
or	O
logical	O
(	O
software	O
)	O
devices	O
.	O
</s>
<s>
Because	O
of	O
the	O
differences	O
existing	O
between	O
the	O
hundreds	O
of	O
various	O
implementations	O
of	O
the	O
Linux	B-Application
OS	I-Application
,	O
executable	O
objects	O
,	O
even	O
though	O
they	O
are	O
compiled	B-Language
,	O
assembled	O
,	O
and	O
linked	O
for	O
running	O
on	O
a	O
specific	O
hardware	O
architecture	O
(	O
that	O
is	O
,	O
they	O
use	O
the	O
ISA	B-General_Concept
of	O
the	O
target	O
hardware	O
)	O
,	O
often	O
cannot	O
run	O
on	O
different	O
Linux	B-Application
Distributions	I-Application
.	O
</s>
<s>
This	O
issue	O
is	O
mainly	O
due	O
to	O
distribution-specific	O
configurations	O
and	O
a	O
set	O
of	O
patches	O
applied	O
to	O
the	O
code	O
of	O
the	O
Linux	B-Application
kernel	B-Operating_System
,	O
differences	O
in	O
system	O
libraries	O
,	O
services	O
(	O
daemons	B-Operating_System
)	O
,	O
filesystem	B-Application
hierarchies	O
,	O
and	O
environment	O
variables	O
.	O
</s>
<s>
The	O
main	O
standard	O
concerning	O
application	O
and	O
binary	O
compatibility	O
of	O
Linux	B-Application
distributions	I-Application
is	O
the	O
Linux	B-Application
Standard	I-Application
Base	I-Application
(	O
LSB	O
)	O
.	O
</s>
<s>
However	O
,	O
the	O
LSB	O
goes	O
beyond	O
what	O
concerns	O
the	O
Linux	B-Application
kernel	B-Operating_System
,	O
because	O
it	O
also	O
defines	O
the	O
desktop	O
specifications	O
,	O
the	O
X	B-Operating_System
libraries	O
and	O
Qt	O
that	O
have	O
little	O
to	O
do	O
with	O
it	O
.	O
</s>
<s>
The	O
LSB	O
version	O
5	O
is	O
built	O
upon	O
several	O
standards	O
and	O
drafts	O
(	O
POSIX	O
,	O
SUS	O
,	O
X/Open	O
,	O
File	B-Application
System	I-Application
Hierarchy	I-Application
(	O
FHS	O
)	O
,	O
and	O
others	O
)	O
.	O
</s>
<s>
The	O
parts	O
of	O
the	O
LSB	O
largely	O
relevant	O
to	O
the	O
kernel	B-Operating_System
are	O
the	O
General	O
ABI	B-Operating_System
(	O
gABI	O
)	O
,	O
especially	O
the	O
System	B-Operating_System
V	I-Operating_System
ABI	I-Operating_System
and	O
the	O
Executable	O
and	O
Linking	B-Application
Format	O
(	O
ELF	O
)	O
,	O
and	O
the	O
Processor	O
Specific	O
ABI	B-Operating_System
(	O
psABI	O
)	O
,	O
for	O
example	O
the	O
Core	O
Specification	O
for	O
X86-64	B-Device
.	O
</s>
<s>
The	O
standard	O
ABI	B-Operating_System
for	O
how	O
x86_64	O
user	O
programs	O
invoke	O
system	B-Operating_System
calls	I-Operating_System
is	O
to	O
load	O
the	O
syscall	B-Operating_System
number	O
into	O
the	O
rax	O
register	O
,	O
and	O
the	O
other	O
parameters	O
into	O
rdi	O
,	O
rsi	O
,	O
rdx	O
,	O
r10	O
,	O
r8	O
,	O
and	O
r9	O
,	O
and	O
finally	O
to	O
put	O
the	O
syscall	B-Operating_System
assembly	B-Language
instruction	O
in	O
the	O
code	O
.	O
</s>
<s>
There	O
are	O
several	O
kernel	B-Operating_System
internal	O
APIs	B-Application
utilized	O
between	O
the	O
different	O
subsystems	O
.	O
</s>
<s>
Some	O
are	O
available	O
only	O
within	O
the	O
kernel	B-Operating_System
subsystems	O
,	O
while	O
a	O
somewhat	O
limited	O
set	O
of	O
in-kernel	O
symbols	O
(	O
i.e.	O
,	O
variables	O
,	O
data	O
structures	O
,	O
and	O
functions	O
)	O
is	O
exposed	O
also	O
to	O
dynamically	O
loadable	O
modules	B-Application
(	O
e.g.	O
,	O
device	B-Application
drivers	I-Application
loaded	O
on	O
demand	O
)	O
whether	O
they	O
're	O
exported	O
with	O
the	O
and	O
macros	O
(	O
the	O
latter	O
reserved	O
to	O
modules	B-Application
released	O
under	O
a	O
GPL-compatible	O
license	O
)	O
.	O
</s>
<s>
Linux	B-Application
provides	O
in-kernel	O
APIs	B-Application
that	O
manipulate	O
data	O
structures	O
(	O
e.g.	O
,	O
linked	B-Data_Structure
lists	I-Data_Structure
,	O
radix	B-Data_Structure
trees	I-Data_Structure
,	O
red-black	O
trees	O
,	O
queues	B-Application
)	O
or	O
perform	O
common	O
routines	O
(	O
e.g.	O
,	O
copy	O
data	O
from	O
and	O
to	O
user	B-Operating_System
space	I-Operating_System
,	O
allocate	O
memory	O
,	O
print	O
lines	O
to	O
the	O
system	O
log	O
,	O
and	O
so	O
on	O
)	O
that	O
have	O
remained	O
stable	O
at	B-Operating_System
least	O
since	O
Linux	B-Application
version	O
2.6	O
.	O
</s>
<s>
In-kernel	O
APIs	B-Application
include	O
libraries	O
of	O
low-level	O
common	O
services	O
used	O
by	O
device	B-Application
drivers	I-Application
:	O
</s>
<s>
SCSI	B-Architecture
Interfaces	O
and	O
libATA	B-Application
respectively	O
,	O
a	O
peer-to-peer	O
packet	O
based	O
communication	O
protocol	O
for	O
storage	O
devices	O
attached	O
to	O
USB	B-Protocol
,	O
SATA	O
,	O
SAS	O
,	O
Fibre	O
Channel	O
,	O
FireWire	O
,	O
ATAPI	O
device	O
,	O
and	O
an	O
in-kernel	O
library	O
to	O
support	O
[S]ATA	O
host	O
controllers	O
and	O
devices	O
.	O
</s>
<s>
The	O
Linux	B-Application
developers	O
chose	O
not	O
to	O
maintain	O
a	O
stable	O
in-kernel	O
ABI	B-Operating_System
.	O
</s>
<s>
Modules	B-Application
compiled	B-Language
for	O
a	O
specific	O
version	O
of	O
the	O
kernel	B-Operating_System
cannot	O
be	O
loaded	O
into	O
another	O
version	O
without	O
being	O
recompiled	B-Language
,	O
assuming	O
that	O
the	O
in-kernel	O
API	B-Application
has	O
remained	O
the	O
same	O
at	B-Operating_System
the	O
source	O
level	O
;	O
otherwise	O
,	O
the	O
module	B-Application
code	O
must	O
also	O
be	O
modified	O
accordingly	O
.	O
</s>
<s>
Linux	B-Application
creates	O
processes	B-Operating_System
by	O
means	O
of	O
the	O
or	O
by	O
the	O
newer	O
system	B-Operating_System
calls	I-Operating_System
.	O
</s>
<s>
These	O
syscalls	B-Operating_System
can	O
create	O
new	O
entities	O
ranging	O
from	O
new	O
independent	O
processes	B-Operating_System
(	O
each	O
having	O
a	O
special	O
identifier	O
called	O
TGID	O
within	O
the	O
task_struct	O
data	O
structure	O
in	O
kernel	B-Operating_System
space	I-Operating_System
,	O
although	O
that	O
same	O
identifier	O
is	O
called	O
PID	O
in	O
userspace	B-Operating_System
)	O
,	O
to	O
new	O
threads	B-Operating_System
of	I-Operating_System
execution	I-Operating_System
within	O
the	O
calling	O
process	O
(	O
by	O
using	O
the	O
parameter	O
)	O
.	O
</s>
<s>
In	O
this	O
latter	O
case	O
the	O
new	O
entity	O
owns	O
the	O
same	O
TGID	O
of	O
the	O
calling	O
process	O
and	O
consequently	O
has	O
also	O
the	O
same	O
PID	O
in	O
userspace	B-Operating_System
.	O
</s>
<s>
If	O
the	O
executable	O
is	O
dynamically	O
linked	O
to	O
shared	O
libraries	O
,	O
a	O
dynamic	B-Application
linker	I-Application
(	O
for	O
ELF	O
objects	O
,	O
it	O
is	O
typically	O
)	O
is	O
used	O
to	O
find	O
and	O
load	O
the	O
needed	O
objects	O
,	O
prepare	O
the	O
program	O
to	O
run	O
and	O
then	O
run	O
it	O
.	O
</s>
<s>
The	O
Native	B-Application
POSIX	I-Application
Thread	I-Application
Library	I-Application
,	O
simply	O
known	O
as	O
the	O
NPTL	B-Application
,	O
provides	O
the	O
standard	O
POSIX	O
threads	B-Operating_System
interface	O
(	O
pthreads	O
)	O
to	O
userspace	B-Operating_System
.	O
</s>
<s>
Whenever	O
a	O
new	O
thread	B-Operating_System
is	O
created	O
using	O
the	O
pthread_create(3 )	O
POSIX	O
interface	O
,	O
the	O
family	O
of	O
system	B-Operating_System
calls	I-Operating_System
must	O
also	O
be	O
given	O
the	O
address	O
of	O
the	O
function	O
that	O
the	O
new	O
thread	B-Operating_System
must	O
jump	O
to	O
.	O
</s>
<s>
The	O
Linux	B-Application
kernel	B-Operating_System
provides	O
the	O
(	O
acronym	O
for	O
"	O
Fast	O
user-space	B-Operating_System
mutexes	B-Operating_System
"	O
)	O
mechanisms	O
for	O
fast	O
user-space	B-Operating_System
locking	O
and	O
synchronization	O
;	O
the	O
majority	O
of	O
the	O
operations	O
are	O
performed	O
in	O
userspace	B-Operating_System
but	O
it	O
may	O
be	O
necessary	O
to	O
communicate	O
with	O
the	O
kernel	B-Operating_System
using	O
the	O
system	B-Operating_System
call	I-Operating_System
.	O
</s>
<s>
A	O
very	O
special	O
category	O
of	O
threads	B-Operating_System
is	O
the	O
so-called	O
kernel	B-Operating_System
threads	B-Operating_System
.	O
</s>
<s>
They	O
must	O
not	O
be	O
confused	O
with	O
the	O
above-mentioned	O
threads	B-Operating_System
of	I-Operating_System
execution	I-Operating_System
of	O
the	O
user	O
's	O
processes	B-Operating_System
.	O
</s>
<s>
Kernel	B-Operating_System
threads	B-Operating_System
exist	O
only	O
in	O
kernel	B-Operating_System
space	I-Operating_System
and	O
their	O
only	O
purpose	O
is	O
to	O
concurrently	O
run	O
kernel	B-Operating_System
tasks	O
.	O
</s>
<s>
Differently	O
,	O
whenever	O
an	O
independent	O
process	O
is	O
created	O
,	O
the	O
syscalls	B-Operating_System
return	O
exactly	O
to	O
the	O
next	O
instruction	O
of	O
the	O
same	O
program	O
,	O
concurrently	O
in	O
parent	O
process	O
and	O
in	O
child	O
's	O
one	O
(	O
i.e.	O
,	O
one	O
program	O
,	O
two	O
processes	B-Operating_System
)	O
.	O
</s>
<s>
Different	O
return	O
values	O
(	O
one	O
per	O
process	O
)	O
enable	O
the	O
program	O
to	O
know	O
in	O
which	O
of	O
the	O
two	O
processes	B-Operating_System
it	O
is	O
currently	O
executing	O
.	O
</s>
<s>
Programs	O
need	O
this	O
information	O
because	O
the	O
child	O
process	O
,	O
a	O
few	O
steps	O
after	O
process	O
duplication	O
,	O
usually	O
invokes	O
the	O
system	B-Operating_System
call	I-Operating_System
(	O
possibly	O
via	O
the	O
family	O
of	O
wrapper	O
functions	O
in	O
glibC	B-Language
)	O
and	O
replace	O
the	O
program	O
that	O
is	O
currently	O
being	O
run	O
by	O
the	O
calling	O
process	O
with	O
a	O
new	O
program	O
,	O
with	O
newly	O
initialized	O
stack	O
,	O
heap	O
,	O
and	O
(	O
initialized	O
and	O
uninitialized	O
)	O
data	O
segments	O
.	O
</s>
<s>
When	O
it	O
is	O
done	O
,	O
it	O
results	O
in	O
two	O
processes	B-Operating_System
that	O
run	O
two	O
different	O
programs	O
.	O
</s>
<s>
Depending	O
on	O
the	O
effective	B-Application
user	I-Application
id	I-Application
(	O
euid	O
)	O
,	O
and	O
on	O
the	O
effective	B-Application
group	I-Application
id	I-Application
(	O
egid	O
)	O
,	O
a	O
process	O
running	O
with	O
user	O
zero	O
privileges	O
(	O
root	O
,	O
the	O
system	O
administrator	O
,	O
owns	O
the	O
identifier	O
0	O
)	O
can	O
perform	O
everything	O
(	O
e.g.	O
,	O
kill	O
all	O
the	O
other	O
processes	B-Operating_System
or	O
recursively	O
wipe	O
out	O
whole	O
filesystems	B-Application
)	O
,	O
instead	O
non	O
zero	O
user	O
processes	B-Operating_System
cannot	O
.	O
</s>
<s>
The	O
Linux	B-Application
scheduler	O
is	O
modular	B-Architecture
,	O
in	O
the	O
sense	O
that	O
it	O
enables	O
different	O
scheduling	O
classes	O
and	O
policies	O
.	O
</s>
<s>
Each	O
class	O
schedules	O
different	O
types	O
of	O
processes	B-Operating_System
.	O
</s>
<s>
Entities	O
may	O
be	O
threads	B-Operating_System
,	O
group	O
of	O
threads	B-Operating_System
,	O
and	O
even	O
all	O
the	O
processes	B-Operating_System
of	O
a	O
specific	O
user	O
.	O
</s>
<s>
Linux	B-Application
provides	O
both	O
user	O
preemption	B-Operating_System
as	O
well	O
as	O
full	O
kernel	B-Operating_System
preemption	B-Operating_System
.	O
</s>
<s>
Preemption	B-Operating_System
reduces	O
latency	O
,	O
increases	O
responsiveness	O
,	O
and	O
makes	O
Linux	B-Application
more	O
suitable	O
for	O
desktop	O
and	O
real-time	B-General_Concept
applications	I-General_Concept
.	O
</s>
<s>
For	O
normal	O
tasks	O
,	O
by	O
default	O
,	O
the	O
kernel	B-Operating_System
uses	O
the	O
Completely	B-Application
Fair	I-Application
Scheduler	I-Application
(	O
CFS	B-Application
)	O
class	O
,	O
introduced	O
in	O
the	O
2.6.23	O
version	O
of	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
Internally	O
this	O
default-scheduler	O
class	O
is	O
defined	O
in	O
a	O
macro	O
of	O
a	O
C	B-Language
header	I-Language
as	O
SCHED_NORMAL	O
.	O
</s>
<s>
In	O
other	O
POSIX	O
kernels	B-Operating_System
,	O
a	O
similar	O
policy	O
known	O
as	O
SCHED_OTHER	O
allocates	O
CPU	B-Device
timeslices	O
(	O
i.e	O
,	O
it	O
assigns	O
absolute	O
slices	O
of	O
the	O
processor	O
time	O
depending	O
on	O
either	O
predetermined	O
or	O
dynamically	O
computed	O
priority	O
of	O
each	O
process	O
)	O
.	O
</s>
<s>
The	O
Linux	B-Application
CFS	B-Application
does	O
away	O
with	O
absolute	O
timeslices	O
and	O
assigns	O
a	O
fair	O
proportion	O
of	O
CPU	B-Device
time	O
,	O
as	O
a	O
function	O
of	O
parameters	O
like	O
the	O
total	O
number	O
of	O
runnable	O
processes	B-Operating_System
and	O
the	O
time	O
they	O
have	O
already	O
run	O
;	O
this	O
function	O
also	O
takes	O
into	O
account	O
a	O
kind	O
of	O
weight	O
that	O
depends	O
on	O
their	O
relative	O
priorities	O
(	O
nice	O
values	O
)	O
.	O
</s>
<s>
With	O
user	O
preemption	B-Operating_System
,	O
the	O
kernel	B-Operating_System
scheduler	O
can	O
replace	O
the	O
current	O
process	O
with	O
the	O
execution	O
of	O
a	O
context	B-Operating_System
switch	I-Operating_System
to	O
a	O
different	O
one	O
that	O
therefore	O
acquires	O
the	O
computing	O
resources	O
for	O
running	O
(	O
CPU	B-Device
,	O
memory	O
,	O
and	O
more	O
)	O
.	O
</s>
<s>
It	O
makes	O
it	O
according	O
to	O
the	O
CFS	B-Application
algorithm	O
(	O
in	O
particular	O
,	O
it	O
uses	O
a	O
variable	O
called	O
for	O
sorting	O
entities	O
and	O
then	O
chooses	O
the	O
one	O
that	O
has	O
the	O
smaller	O
vruntime	O
,	O
-	O
i.e.	O
,	O
the	O
schedulable	O
entity	O
that	O
has	O
had	O
the	O
least	O
share	O
of	O
CPU	B-Device
time	O
)	O
,	O
to	O
the	O
active	O
scheduler	O
policy	O
and	O
to	O
the	O
relative	O
priorities	O
.	O
</s>
<s>
With	O
kernel	B-Operating_System
preemption	B-Operating_System
,	O
the	O
kernel	B-Operating_System
can	O
preempt	O
itself	O
when	O
an	O
interrupt	B-General_Concept
handler	I-General_Concept
returns	O
,	O
when	O
kernel	B-Operating_System
tasks	O
block	O
,	O
and	O
whenever	O
a	O
subsystem	O
explicitly	O
calls	O
the	O
schedule( )	O
function	O
.	O
</s>
<s>
The	O
kernel	B-Operating_System
also	O
contains	O
two	O
POSIX-compliant	O
real-time	B-General_Concept
scheduling	O
classes	O
named	O
SCHED_FIFO	O
(	O
realtime	O
first-in-first-out	B-Operating_System
)	O
and	O
SCHED_RR	O
(	O
realtime	O
round-robin	B-Algorithm
)	O
,	O
both	O
of	O
which	O
take	O
precedence	O
over	O
the	O
default	O
class	O
.	O
</s>
<s>
An	O
additional	O
scheduling	O
policy	O
known	O
as	O
SCHED	B-Application
DEADLINE	I-Application
,	O
implementing	O
the	O
earliest	B-Operating_System
deadline	I-Operating_System
first	I-Operating_System
algorithm	I-Operating_System
(	O
EDF	O
)	O
,	O
was	O
added	O
in	O
kernel	B-Operating_System
version	O
3.14	O
,	O
released	O
on	O
30	O
March	O
2014	O
.	O
</s>
<s>
Real-time	B-General_Concept
PREEMPT_RT	O
patches	O
,	O
included	O
into	O
the	O
mainline	B-Application
Linux	I-Application
since	O
version	O
2.6	O
,	O
provide	O
a	O
deterministic	O
scheduler	O
,	O
the	O
removal	O
of	O
preemption	B-Operating_System
and	O
interrupts	B-Application
disabling	O
(	O
where	O
possible	O
)	O
,	O
PI	O
Mutexes	B-Operating_System
(	O
i.e.	O
,	O
locking	O
primitives	O
that	O
avoid	O
priority	O
inversion	O
)	O
,	O
support	O
for	O
high	O
precision	O
event	O
timers	O
(	O
HPET	O
)	O
,	O
preemptive	O
Read-copy-update	B-Operating_System
,	O
(	O
forced	O
)	O
IRQ	O
threads	B-Operating_System
,	O
and	O
other	O
minor	O
features	O
.	O
</s>
<s>
The	O
kernel	B-Operating_System
has	O
different	O
causes	O
of	O
concurrency	B-Architecture
(	O
e.g.	O
,	O
interrupts	B-Application
,	O
bottom	O
halves	O
,	O
preemption	B-Operating_System
of	O
kernel	B-Operating_System
and	O
users	O
tasks	O
,	O
symmetrical	B-Operating_System
multiprocessing	I-Operating_System
)	O
.	O
</s>
<s>
For	O
protecting	O
critical	O
regions	O
(	O
sections	O
of	O
code	O
that	O
must	O
be	O
executed	O
atomically	O
)	O
,	O
shared	O
memory	O
locations	O
(	O
like	O
global	O
variables	O
and	O
other	O
data	O
structures	O
with	O
global	O
scope	O
)	O
,	O
and	O
regions	O
of	O
memory	O
that	O
are	O
asynchronously	O
modifiable	O
by	O
hardware	O
(	O
e.g.	O
,	O
having	O
the	O
C	B-Language
volatile	B-Operating_System
type	B-Language
qualifier	I-Language
)	O
,	O
Linux	B-Application
provides	O
a	O
large	O
set	O
of	O
tools	O
.	O
</s>
<s>
They	O
consist	O
of	O
atomic	B-General_Concept
types	I-General_Concept
(	O
which	O
can	O
only	O
be	O
manipulated	O
by	O
a	O
set	O
of	O
specific	O
operators	O
)	O
,	O
spinlocks	B-Operating_System
,	O
semaphores	B-Operating_System
,	O
mutexes	B-Operating_System
,	O
and	O
lockless	B-Operating_System
algorithms	I-Operating_System
(	O
e.g.	O
,	O
RCUs	B-Operating_System
)	O
.	O
</s>
<s>
Most	O
lock-less	O
algorithms	O
are	O
built	O
on	O
top	O
of	O
memory	B-General_Concept
barriers	I-General_Concept
for	O
the	O
purpose	O
of	O
enforcing	O
memory	B-General_Concept
ordering	I-General_Concept
and	O
prevent	O
undesired	O
side	O
effects	O
due	O
to	O
compiler	B-Application
optimization	I-Application
.	O
</s>
<s>
PREEMPT_RT	O
code	O
included	O
in	O
mainline	B-Application
Linux	I-Application
provide	O
RT-mutexes	O
,	O
a	O
special	O
kind	O
of	O
Mutex	B-Operating_System
which	O
do	O
not	O
disable	O
preemption	B-Operating_System
and	O
have	O
support	O
for	O
priority	B-Operating_System
inheritance	I-Operating_System
.	O
</s>
<s>
Priority	B-Operating_System
inheritance	I-Operating_System
avoids	O
priority	O
inversion	O
by	O
granting	O
a	O
low-priority	O
task	O
which	O
holds	O
a	O
contended	O
lock	O
the	O
priority	O
of	O
a	O
higher-priority	O
waiter	O
until	O
that	O
lock	O
is	O
released	O
.	O
</s>
<s>
Linux	B-Application
includes	O
a	O
kernel	B-Operating_System
lock	O
validator	O
called	O
Lockdep	O
.	O
</s>
<s>
The	O
management	O
of	O
the	O
interrupts	B-Application
,	O
although	O
it	O
could	O
be	O
seen	O
as	O
a	O
single	O
job	O
,	O
is	O
divided	O
in	O
two	O
separate	O
parts	O
.	O
</s>
<s>
The	O
first	O
part	O
is	O
made	O
up	O
of	O
an	O
asynchronous	O
interrupt	B-General_Concept
service	I-General_Concept
routine	I-General_Concept
that	O
in	O
Linux	B-Application
is	O
known	O
as	O
the	O
top	O
half	O
,	O
while	O
the	O
second	O
part	O
is	O
carried	O
out	O
by	O
one	O
of	O
three	O
types	O
of	O
the	O
so-called	O
bottom	O
halves	O
(	O
softirq	O
,	O
tasklets	O
,	O
and	O
work	O
queues	B-Application
)	O
.	O
</s>
<s>
Linux	B-Application
interrupts	B-Application
service	O
routines	O
can	O
be	O
nested	O
(	O
i.e.	O
,	O
a	O
new	O
IRQ	O
can	O
trap	B-Application
into	O
a	O
high	O
priority	O
ISR	O
that	O
preempts	O
any	O
other	O
lower	O
priority	O
ISRs	O
)	O
.	O
</s>
<s>
Memory	B-General_Concept
management	I-General_Concept
in	O
Linux	B-Application
is	O
a	O
complex	O
topic	O
.	O
</s>
<s>
First	O
of	O
all	O
,	O
the	O
kernel	B-Operating_System
is	O
not	O
pageable	B-Architecture
(	O
i.e.	O
,	O
it	O
is	O
always	O
resident	O
in	O
physical	O
memory	O
and	O
cannot	O
be	O
swapped	O
to	O
the	O
disk	O
)	O
.	O
</s>
<s>
In	O
the	O
kernel	B-Operating_System
there	O
is	O
no	O
memory	B-General_Concept
protection	I-General_Concept
(	O
no	O
SIGSEGV	O
signals	O
,	O
unlike	O
in	O
userspace	B-Operating_System
)	O
,	O
therefore	O
memory	O
violations	O
lead	O
to	O
instability	O
and	O
system	O
crashes	O
.	O
</s>
<s>
Linux	B-Application
implements	O
virtual	B-Architecture
memory	I-Architecture
with	O
4	O
and	O
5-levels	O
page	B-General_Concept
tables	I-General_Concept
.	O
</s>
<s>
As	O
said	O
,	O
only	O
user	O
memory	O
space	O
is	O
always	O
pageable	B-Architecture
.	O
</s>
<s>
It	O
maintains	O
information	O
about	O
each	O
page	B-General_Concept
frame	I-General_Concept
of	O
RAM	B-Architecture
in	O
apposite	O
data	O
structures	O
(	O
of	O
type	O
)	O
that	O
are	O
populated	O
immediately	O
after	O
boots	O
and	O
that	O
are	O
kept	O
until	O
shutdown	O
,	O
regardless	O
of	O
them	O
being	O
or	O
not	O
associated	O
with	O
virtual	B-General_Concept
pages	I-General_Concept
.	O
</s>
<s>
Furthermore	O
,	O
it	O
classifies	O
all	O
page	B-General_Concept
frames	I-General_Concept
in	O
zones	O
,	O
according	O
to	O
their	O
architecture	O
dependent	O
constraints	O
and	O
intended	O
use	O
.	O
</s>
<s>
For	O
example	O
,	O
pages	O
reserved	O
for	O
DMA	B-General_Concept
operations	O
are	O
in	O
ZONE_DMA	O
,	O
pages	O
that	O
are	O
not	O
permanently	O
mapped	O
to	O
virtual	O
addresses	O
are	O
in	O
ZONE_HIGHMEM	O
(	O
in	O
x86_32	O
architecture	O
this	O
zone	O
is	O
for	O
physical	O
addresses	O
above	O
896	O
MB	O
,	O
while	O
x86_64	O
does	O
not	O
need	O
it	O
because	O
x86_64	O
can	O
permanently	O
map	O
physical	O
pages	O
that	O
reside	O
in	O
higher	O
addresses	O
)	O
,	O
and	O
all	O
that	O
remains	O
(	O
with	O
the	O
exception	O
of	O
other	O
less	O
used	O
classifications	O
)	O
is	O
in	O
ZONE_NORMAL	O
.	O
</s>
<s>
Small	O
chunks	O
of	O
memory	O
can	O
be	O
dynamically	O
allocated	O
via	O
the	O
family	O
of	O
kmalloc( )	O
API	B-Application
and	O
freed	O
with	O
the	O
appropriate	O
variant	O
of	O
kfree( )	O
.	O
</s>
<s>
Kernel	B-Operating_System
includes	O
SLAB	O
,	O
SLUB	O
and	O
SLOB	O
allocators	O
as	O
configurable	O
alternatives	O
.	O
</s>
<s>
While	O
not	O
originally	O
designed	O
to	O
be	O
portable	O
,	O
Linux	B-Application
is	O
now	O
one	O
of	O
the	O
most	O
widely	O
ported	O
operating	B-Operating_System
system	I-Operating_System
kernels	I-Operating_System
,	O
running	O
on	O
a	O
diverse	O
range	O
of	O
systems	O
from	O
the	O
ARM	B-Architecture
architecture	I-Architecture
to	O
IBM	B-Device
z/Architecture	I-Device
mainframe	B-Architecture
computers	I-Architecture
.	O
</s>
<s>
The	O
first	O
port	O
was	O
performed	O
on	O
the	O
Motorola	B-Device
68000	I-Device
platform	O
.	O
</s>
<s>
The	O
modifications	O
to	O
the	O
kernel	B-Operating_System
were	O
so	O
fundamental	O
that	O
Torvalds	O
viewed	O
the	O
Motorola	O
version	O
as	O
a	O
fork	B-Application
and	O
a	O
"	O
Linux-like	O
operating	O
system	O
"	O
.	O
</s>
<s>
However	O
,	O
that	O
moved	O
Torvalds	O
to	O
lead	O
a	O
major	O
restructure	O
of	O
the	O
code	O
to	O
facilitate	O
porting	O
to	O
more	O
computing	O
architectures	B-General_Concept
.	O
</s>
<s>
The	O
first	O
Linux	B-Application
that	O
,	O
in	O
a	O
single	O
source	O
tree	O
,	O
had	O
code	O
for	O
more	O
than	O
i386	B-General_Concept
alone	O
,	O
supported	O
the	O
DEC	B-Device
Alpha	I-Device
AXP	O
64-bit	B-Device
platform	O
.	O
</s>
<s>
Linux	B-Application
runs	O
as	O
the	O
main	O
operating	O
system	O
on	O
IBM	O
's	O
Summit	B-Device
;	O
,	O
all	O
of	O
the	O
world	O
's	O
500	B-Operating_System
fastest	I-Operating_System
supercomputers	I-Operating_System
run	O
some	O
operating	O
system	O
based	O
on	O
the	O
Linux	B-Application
kernel	B-Operating_System
,	O
a	O
big	O
change	O
from	O
1998	O
when	O
the	O
first	O
Linux	B-Application
supercomputer	B-Architecture
got	O
added	O
to	O
the	O
list	O
.	O
</s>
<s>
Linux	B-Application
has	O
also	O
been	O
ported	O
to	O
various	O
handheld	B-Application
devices	I-Application
such	O
as	O
Apple	O
's	O
iPhone	B-Device
3G	O
and	O
iPod	B-Device
.	O
</s>
<s>
In	O
2007	O
,	O
the	O
LKDDb	O
project	O
has	O
been	O
started	O
to	O
build	O
a	O
comprehensive	O
database	O
of	O
hardware	O
and	O
protocols	O
known	O
by	O
Linux	B-Application
kernels	B-Operating_System
.	O
</s>
<s>
The	O
database	O
is	O
built	O
automatically	O
by	O
static	O
analysis	O
of	O
the	O
kernel	B-Operating_System
sources	O
.	O
</s>
<s>
Later	O
in	O
2014	O
,	O
the	O
Linux	B-Application
Hardware	O
project	O
was	O
launched	O
to	O
automatically	O
collect	O
a	O
database	O
of	O
all	O
tested	O
hardware	O
configurations	O
with	O
the	O
help	O
of	O
users	O
of	O
various	O
Linux	B-Application
distributions	I-Application
.	O
</s>
<s>
Rebootless	O
updates	O
can	O
even	O
be	O
applied	O
to	O
the	O
kernel	B-Operating_System
by	O
using	O
live	O
patching	O
technologies	O
such	O
as	O
Ksplice	B-Application
,	O
kpatch	B-Application
and	O
kGraft	B-Application
.	O
</s>
<s>
Minimalistic	O
foundations	O
for	O
live	O
kernel	B-Operating_System
patching	O
were	O
merged	O
into	O
the	O
Linux	B-Application
kernel	B-Operating_System
mainline	O
in	O
kernel	B-Operating_System
version	O
4.0	O
,	O
which	O
was	O
released	O
on	O
12	O
April	O
2015	O
.	O
</s>
<s>
Those	O
foundations	O
,	O
known	O
as	O
livepatch	O
and	O
based	O
primarily	O
on	O
the	O
kernel	B-Operating_System
's	O
ftrace	B-Application
functionality	O
,	O
form	O
a	O
common	O
core	O
capable	O
of	O
supporting	O
hot	O
patching	O
by	O
both	O
kGraft	B-Application
and	O
kpatch	B-Application
,	O
by	O
providing	O
an	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-Application
)	O
for	O
kernel	B-Application
modules	I-Application
that	O
contain	O
hot	O
patches	O
and	O
an	O
application	B-Operating_System
binary	I-Operating_System
interface	I-Operating_System
(	O
ABI	B-Operating_System
)	O
for	O
the	O
userspace	B-Operating_System
management	O
utilities	B-Application
.	O
</s>
<s>
However	O
,	O
the	O
common	O
core	O
included	O
into	O
Linux	B-Application
kernel	B-Operating_System
4.0	O
supports	O
only	O
the	O
x86	B-Operating_System
architecture	I-Operating_System
and	O
does	O
not	O
provide	O
any	O
mechanisms	O
for	O
ensuring	O
function-level	O
consistency	O
while	O
the	O
hot	O
patches	O
are	O
applied	O
.	O
</s>
<s>
,	O
there	O
is	O
ongoing	O
work	O
on	O
porting	O
kpatch	B-Application
and	O
kGraft	B-Application
to	O
the	O
common	O
live	O
patching	O
core	O
provided	O
by	O
the	O
Linux	B-Application
kernel	B-Operating_System
mainline	O
.	O
</s>
<s>
Kernel	B-Operating_System
bugs	B-Error_Name
present	O
potential	O
security	O
issues	O
.	O
</s>
<s>
Over	O
the	O
years	O
,	O
numerous	O
bugs	B-Error_Name
affecting	O
system	O
security	O
were	O
found	O
and	O
fixed	O
.	O
</s>
<s>
New	O
features	O
are	O
frequently	O
implemented	O
to	O
improve	O
the	O
kernel	B-Operating_System
's	O
security	O
.	O
</s>
<s>
Capabilities(7 )	O
have	O
already	O
been	O
introduced	O
in	O
the	O
section	O
about	O
the	O
processes	B-Operating_System
and	O
threads	B-Operating_System
.	O
</s>
<s>
Android	B-Application
makes	O
use	O
of	O
them	O
and	O
systemd	B-Application
gives	O
administrators	O
detailed	O
control	O
over	O
the	O
capabilities	O
of	O
processes	B-Operating_System
.	O
</s>
<s>
Linux	B-Application
offers	O
a	O
wealth	O
of	O
mechanisms	O
to	O
reduce	O
kernel	B-Operating_System
attack	O
surface	O
and	O
improve	O
security	O
which	O
are	O
collectively	O
known	O
as	O
the	O
Linux	B-Application
Security	I-Application
Modules	I-Application
(	O
LSM	O
)	O
.	O
</s>
<s>
They	O
comprise	O
the	O
Security-Enhanced	B-Application
Linux	I-Application
(	O
SELinux	B-Application
)	O
module	B-Application
,	O
whose	O
code	O
has	O
been	O
originally	O
developed	O
and	O
then	O
released	O
to	O
the	O
public	O
by	O
the	O
NSA	O
,	O
and	O
AppArmor	O
among	O
others	O
.	O
</s>
<s>
SELinux	B-Application
is	O
now	O
actively	O
developed	O
and	O
maintained	O
on	O
GitHub	B-Application
.	O
</s>
<s>
SELinux	B-Application
and	O
AppArmor	O
provide	O
support	O
to	O
access	O
control	O
security	O
policies	O
,	O
including	O
mandatory	O
access	O
control	O
(	O
MAC	O
)	O
,	O
though	O
they	O
profoundly	O
differ	O
in	O
complexity	O
and	O
scope	O
.	O
</s>
<s>
Another	O
security	O
feature	O
is	O
the	O
Seccomp	O
BPF	O
(	O
SECure	O
COMPuting	O
with	O
Berkeley	O
Packet	O
Filters	O
)	O
which	O
works	O
by	O
filtering	O
parameters	O
and	O
reducing	O
the	O
set	O
of	O
system	B-Operating_System
calls	I-Operating_System
available	O
to	O
user-land	O
applications	O
.	O
</s>
<s>
Critics	O
have	O
accused	O
kernel	B-Operating_System
developers	O
of	O
covering	O
up	O
security	O
flaws	O
,	O
or	O
at	B-Operating_System
least	O
not	O
announcing	O
them	O
;	O
in	O
2008	O
,	O
Linus	O
Torvalds	O
responded	O
to	O
this	O
with	O
the	O
following	O
:	O
</s>
<s>
Linux	B-Application
distributions	I-Application
typically	O
release	O
security	O
updates	O
to	O
fix	O
vulnerabilities	O
in	O
the	O
Linux	B-Application
kernel	B-Operating_System
.	O
</s>
<s>
Many	O
offer	O
long-term	O
support	O
releases	O
that	O
receive	O
security	O
updates	O
for	O
a	O
certain	O
Linux	B-Application
kernel	B-Operating_System
version	O
for	O
an	O
extended	O
period	O
of	O
time	O
.	O
</s>
<s>
The	O
community	O
of	O
Linux	B-Application
kernel	B-Operating_System
developers	O
comprises	O
about	O
5000	O
–	O
6000	O
members	O
.	O
</s>
<s>
According	O
to	O
the	O
"	O
2017	O
State	O
of	O
Linux	B-Application
Kernel	B-Operating_System
Development	O
"	O
,	O
a	O
study	O
issued	O
by	O
the	B-Operating_System
Linux	I-Operating_System
Foundation	I-Operating_System
,	O
covering	O
the	O
commits	O
for	O
the	O
releases	O
4.8	O
to	O
4.13	O
,	O
about	O
1500	O
developers	O
were	O
contributing	O
from	O
about	O
200-250	O
companies	O
on	O
average	O
.	O
</s>
<s>
The	O
Linux	B-Application
development	O
community	O
uses	O
Git	B-Application
to	O
manage	O
the	O
source	O
code	O
.	O
</s>
<s>
Git	B-Application
users	O
clone	O
the	O
latest	O
version	O
of	O
Torvalds	O
 '	O
tree	O
with	O
and	O
keep	O
it	O
up	O
to	O
date	O
using	O
.	O
</s>
<s>
Contributions	O
are	O
submitted	O
as	O
patches	O
,	O
in	O
the	O
form	O
of	O
text	O
messages	O
on	O
the	O
LKML	B-Application
(	O
and	O
often	O
also	O
on	O
other	O
mailing	O
lists	O
dedicated	O
to	O
particular	O
subsystems	O
)	O
.	O
</s>
<s>
These	O
patches	O
can	O
be	O
automatically	O
processed	O
so	O
that	O
system	O
administrators	O
can	O
apply	O
them	O
in	O
order	O
to	O
make	B-Application
just	O
some	O
changes	O
to	O
the	O
code	O
or	O
to	O
incrementally	O
upgrade	O
to	O
the	O
next	O
version	O
.	O
</s>
<s>
Linux	B-Application
is	O
distributed	O
also	O
in	O
GNU	B-Application
zip	I-Application
(	O
gzip	B-Application
)	O
and	O
bzip2	B-Application
formats	O
.	O
</s>
<s>
A	O
developer	O
who	O
wants	O
to	O
change	O
the	O
Linux	B-Application
kernel	B-Operating_System
starts	O
with	O
developing	O
and	O
testing	O
that	O
change	O
.	O
</s>
<s>
Once	O
the	O
review	O
process	O
has	O
finished	O
the	O
subsystem	O
maintainer	O
accepts	O
the	O
patches	O
in	O
the	O
relevant	O
Git	B-Application
kernel	B-Operating_System
tree	O
.	O
</s>
<s>
If	O
the	O
changes	O
to	O
the	O
Linux	B-Application
kernel	B-Operating_System
are	O
bug	B-Error_Name
fixes	I-Error_Name
that	O
are	O
considered	O
important	O
enough	O
,	O
a	O
pull	O
request	O
for	O
the	O
patches	O
will	O
be	O
sent	O
to	O
Torvalds	O
within	O
a	O
few	O
days	O
.	O
</s>
<s>
The	O
merge	O
window	O
usually	O
lasts	O
two	O
weeks	O
and	O
starts	O
immediately	O
after	O
the	O
release	O
of	O
the	O
previous	O
kernel	B-Operating_System
version	O
.	O
</s>
<s>
The	O
Git	B-Application
kernel	B-Operating_System
source	O
tree	O
names	O
all	O
developers	O
who	O
have	O
contributed	O
to	O
the	O
Linux	B-Application
kernel	B-Operating_System
in	O
the	O
Credits	O
directory	O
and	O
all	O
subsystem	O
maintainers	O
are	O
listed	O
in	O
Maintainers	O
.	O
</s>
<s>
Linux	B-Application
is	O
written	O
in	O
a	O
special	O
C	B-Language
programming	I-Language
language	I-Language
supported	O
by	O
GCC	B-Application
,	O
a	O
compiler	B-Language
that	O
extends	O
in	O
many	O
ways	O
the	O
C	B-Language
standard	O
,	O
for	O
example	O
using	O
inline	B-Language
sections	I-Language
of	I-Language
code	I-Language
written	O
in	O
the	O
assembly	B-Language
language	I-Language
(	O
in	O
GCC	B-Application
's	O
"	O
AT&	O
T-style	O
"	O
syntax	O
)	O
of	O
the	O
target	O
architecture	O
.	O
</s>
<s>
Since	O
2002	O
all	O
the	O
code	O
must	O
adhere	O
to	O
the	O
21	O
rules	O
comprising	O
the	O
Linux	B-Application
Kernel	B-Operating_System
Coding	O
Style	O
.	O
</s>
<s>
The	O
GNU	B-Application
Compiler	I-Application
Collection	I-Application
(	O
GCC	B-Application
or	O
GNU	B-Operating_System
cc	O
)	O
is	O
the	O
default	O
compiler	B-Language
for	O
the	O
mainline	B-Application
Linux	I-Application
sources	O
and	O
it	O
is	O
invoked	O
by	O
a	O
utility	O
called	O
make	B-Application
.	O
</s>
<s>
Then	O
,	O
the	O
GNU	B-Application
Assembler	I-Application
(	O
more	O
often	O
called	O
GAS	O
or	O
GNU	B-Application
as	I-Application
)	O
outputs	O
the	O
object	B-Application
files	I-Application
from	O
the	O
GCC	B-Application
generated	O
assembly	B-Language
code	I-Language
.	O
</s>
<s>
Finally	O
,	O
the	O
GNU	B-Application
Linker	I-Application
(	O
GNU	B-Operating_System
ld	O
)	O
is	O
used	O
to	O
produce	O
a	O
statically	O
linked	O
executable	O
kernel	B-Operating_System
file	O
called	O
.	O
</s>
<s>
Both	O
and	O
are	O
part	O
of	O
GNU	B-Application
Binary	I-Application
Utilities	I-Application
(	O
binutils	B-Application
)	O
.	O
</s>
<s>
The	O
above-mentioned	O
tools	O
are	O
collectively	O
known	O
as	O
the	O
GNU	B-Application
toolchain	I-Application
.	O
</s>
<s>
GCC	B-Application
was	O
for	O
a	O
long	O
time	O
the	O
only	O
compiler	B-Language
capable	O
of	O
correctly	O
building	O
Linux	B-Application
.	O
</s>
<s>
In	O
2004	O
,	O
Intel	O
claimed	O
to	O
have	O
modified	O
the	O
kernel	B-Operating_System
so	O
that	O
its	B-Language
C	I-Language
compiler	I-Language
was	O
also	O
capable	O
of	O
compiling	B-Language
it	O
.	O
</s>
<s>
Support	O
for	O
the	O
Intel	O
compiler	B-Language
has	O
been	O
dropped	O
in	O
2023	O
.	O
</s>
<s>
Since	O
2010	O
,	O
effort	O
has	O
been	O
underway	O
to	O
build	O
Linux	B-Application
with	O
Clang	B-Application
,	O
an	O
alternative	O
compiler	B-Language
for	O
the	O
C	B-Language
language	I-Language
;	O
as	O
of	O
12	O
April	O
2014	O
,	O
the	O
official	O
kernel	B-Operating_System
could	O
almost	O
be	O
compiled	B-Language
by	O
Clang	B-Application
.	O
</s>
<s>
The	O
project	O
dedicated	O
to	O
this	O
effort	O
is	O
named	O
LLVMLinux	O
after	O
the	O
LLVM	B-Application
compiler	B-Language
infrastructure	O
upon	O
which	O
Clang	B-Application
is	O
built	O
.	O
</s>
<s>
LLVMLinux	O
does	O
not	O
aim	O
to	O
fork	B-Application
either	O
Linux	B-Application
or	O
the	O
LLVM	B-Application
,	O
therefore	O
it	O
is	O
a	O
meta-project	O
composed	O
of	O
patches	O
that	O
are	O
eventually	O
submitted	O
to	O
the	O
upstream	O
projects	O
.	O
</s>
<s>
By	O
enabling	O
Linux	B-Application
to	O
be	O
compiled	B-Language
by	O
Clang	B-Application
,	O
developers	O
may	O
benefit	O
from	O
shorter	O
compilation	B-Language
times	O
.	O
</s>
<s>
In	O
2017	O
,	O
developers	O
completed	O
upstreaming	O
patches	O
to	O
support	O
building	O
the	O
Linux	B-Application
kernel	B-Operating_System
with	O
Clang	B-Application
in	O
the	O
4.15	O
release	O
,	O
having	O
backported	O
support	O
for	O
X86-64	B-Device
and	O
AArch64	B-Architecture
to	O
the	O
4.4	O
,	O
4.9	O
,	O
and	O
4.14	O
branches	O
of	O
the	O
stable	O
kernel	B-Operating_System
tree	O
.	O
</s>
<s>
Google	B-Application
's	I-Application
Pixel	B-Device
2	I-Device
shipped	O
with	O
the	O
first	O
Clang	B-Application
built	O
Linux	B-Application
kernel	B-Operating_System
,	O
though	O
patches	O
for	O
Pixel	B-Device
(	O
1st	O
generation	O
)	O
did	O
exist	O
.	O
</s>
<s>
2018	O
saw	O
ChromeOS	B-Operating_System
move	O
to	O
building	O
kernels	B-Operating_System
with	O
Clang	B-Application
by	O
default	O
,	O
while	O
Android	B-Application
(	O
operating	O
system	O
)	O
made	O
Clang	B-Application
and	O
LLVM	B-Application
's	O
linker	B-Application
LLD	O
required	O
for	O
kernel	B-Operating_System
builds	O
in	O
2019	O
.	O
</s>
<s>
Google	B-Application
moved	O
its	O
production	O
kernel	B-Operating_System
used	O
throughout	O
its	O
datacenters	O
to	O
being	O
built	O
with	O
Clang	B-Application
in	O
2020	O
.	O
</s>
<s>
Today	O
,	O
the	O
ClangBuiltLinux	O
group	O
coordinates	O
fixes	O
to	O
both	O
Linux	B-Application
and	O
LLVM	B-Application
to	O
ensure	O
compatibility	O
,	O
both	O
composed	O
of	O
members	O
from	O
LLVMLinux	O
and	O
having	O
upstreamed	O
patches	O
from	O
LLVMLinux	O
.	O
</s>
<s>
Bugs	B-Error_Name
involving	O
the	O
Linux	B-Application
Kernel	B-Operating_System
can	O
be	O
difficult	O
to	O
troubleshoot	O
.	O
</s>
<s>
This	O
is	O
because	O
of	O
the	O
kernel	B-Operating_System
's	O
interaction	O
with	O
userspace	B-Operating_System
and	O
hardware	O
;	O
and	O
also	O
because	O
they	O
might	O
be	O
caused	O
from	O
a	O
wider	O
range	O
of	O
reasons	O
compared	O
to	O
those	O
of	O
user	O
programs	O
.	O
</s>
<s>
A	O
report	O
of	O
a	O
non-fatal	O
bug	B-Error_Name
in	O
the	O
kernel	B-Operating_System
is	O
called	O
an	O
"	O
oops	B-Application
"	O
;	O
such	O
deviations	O
from	O
correct	O
behavior	O
of	O
the	O
Linux	B-Application
kernel	B-Operating_System
may	O
allow	O
continued	O
operation	O
with	O
compromised	O
reliability	O
.	O
</s>
<s>
It	O
prints	O
a	O
message	O
and	O
then	O
halts	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
One	O
of	O
the	O
most	O
common	O
techniques	O
used	O
to	O
find	O
out	O
bugs	B-Error_Name
in	O
code	O
is	O
debugging	O
by	O
printing	O
.	O
</s>
<s>
For	O
this	O
purpose	O
Linux	B-Application
provides	O
an	O
in-kernel	O
API	B-Application
called	O
which	O
stores	O
messages	O
in	O
a	O
circular	O
buffer	O
.	O
</s>
<s>
The	O
system	B-Operating_System
call	I-Operating_System
is	O
used	O
for	O
reading	O
and/or	O
clearing	O
the	O
kernel	B-Operating_System
message	O
ring	O
buffer	O
and	O
for	O
setting	O
the	O
maximum	O
log	O
level	O
of	O
the	O
messages	O
to	O
be	O
sent	O
to	O
the	O
console	O
(	O
i.e.	O
,	O
one	O
of	O
the	O
eight	O
parameters	O
of	O
,	O
which	O
tell	O
the	O
severity	O
of	O
the	O
condition	O
reported	O
)	O
;	O
usually	O
it	O
is	O
invoked	O
via	O
the	O
glibC	B-Language
wrapper	O
.	O
</s>
<s>
Kernel	B-Operating_System
messages	O
are	O
also	O
exported	O
to	O
userland	O
through	O
the	O
/dev/kmsg	O
interface	O
(	O
e.g.	O
,	O
systemd-journald	B-Application
reads	O
that	O
interface	O
and	O
by	O
default	O
append	O
the	O
messages	O
to	O
)	O
.	O
</s>
<s>
Another	O
fundamental	O
technique	O
for	O
debugging	O
a	O
running	O
kernel	B-Operating_System
is	O
tracing	O
.	O
</s>
<s>
The	O
ftrace	B-Application
mechanism	O
is	O
a	O
Linux	B-Application
internal	O
tracer	O
;	O
it	O
is	O
used	O
for	O
monitoring	O
and	O
debugging	O
Linux	B-Application
at	B-Operating_System
runtime	O
and	O
it	O
can	O
also	O
analyze	O
user	B-Operating_System
space	I-Operating_System
latencies	O
due	O
to	O
kernel	B-Operating_System
misbehavior	O
.	O
</s>
<s>
Furthermore	O
,	O
ftrace	B-Application
allows	O
users	O
to	O
trace	O
Linux	B-Application
at	B-Operating_System
boot-time	O
.	O
</s>
<s>
kprobes	O
and	O
kretprobes	O
can	O
break	O
(	O
like	O
debuggers	O
in	O
userspace	B-Operating_System
)	O
into	O
Linux	B-Application
and	O
non-disruptively	O
collect	O
information	O
.	O
</s>
<s>
kprobes	O
can	O
be	O
inserted	O
into	O
code	O
at	B-Operating_System
(	O
almost	O
)	O
any	O
address	O
,	O
while	O
kretprobes	O
work	O
at	B-Operating_System
function	O
return	O
.	O
</s>
<s>
With	O
KGDB	B-Application
Linux	B-Application
can	O
be	O
debugged	O
in	O
much	O
the	O
same	O
way	O
as	O
userspace	B-Operating_System
programs	O
.	O
</s>
<s>
KGDB	B-Application
requires	O
an	O
additional	O
machine	O
that	O
runs	O
GDB	O
and	O
that	O
is	O
connected	O
to	O
the	O
target	O
to	O
be	O
debugged	O
using	O
a	O
serial	O
cable	O
or	O
Ethernet	O
.	O
</s>
<s>
The	O
Linux	B-Application
kernel	B-Operating_System
project	O
integrates	O
new	O
code	O
on	O
a	O
rolling	O
basis	O
.	O
</s>
<s>
Software	O
checked	O
into	O
the	O
project	O
must	O
work	O
and	O
compile	B-Language
without	O
error	O
.	O
</s>
<s>
Each	O
kernel	B-Operating_System
subsystem	O
is	O
assigned	O
a	O
maintainer	O
who	O
is	O
responsible	O
for	O
reviewing	O
patches	O
against	O
the	O
kernel	B-Operating_System
code	O
standards	O
and	O
keeps	O
a	O
queue	B-Application
of	O
patches	O
that	O
can	O
be	O
submitted	O
to	O
Linus	O
Torvalds	O
within	O
a	O
merge	O
window	O
of	O
several	O
weeks	O
.	O
</s>
<s>
Patches	O
are	O
merged	O
by	O
Torvalds	O
into	O
the	O
source	O
code	O
of	O
the	O
prior	O
stable	O
Linux	B-Application
kernel	B-Operating_System
release	O
,	O
creating	O
the	O
-rc	O
release	O
candidate	O
for	O
the	O
next	O
stable	O
kernel	B-Operating_System
.	O
</s>
<s>
The	O
-rc	O
development	O
release	O
of	O
the	O
kernel	B-Operating_System
goes	O
through	O
regression	O
tests	O
and	O
once	O
it	O
is	O
judged	O
to	O
be	O
stable	O
by	O
Torvalds	O
and	O
the	O
kernel	B-Operating_System
subsystem	O
maintainers	O
a	O
new	O
Linux	B-Application
kernel	B-Operating_System
is	O
released	O
and	O
the	O
development	O
process	O
starts	O
all	O
over	O
again	O
.	O
</s>
<s>
Developers	O
who	O
feel	O
treated	O
unfairly	O
can	O
report	O
this	O
to	O
the	B-Operating_System
Linux	I-Operating_System
Foundation	I-Operating_System
's	O
Technical	O
Advisory	O
Board	O
.	O
</s>
<s>
In	O
July	O
2013	O
,	O
the	O
maintainer	O
of	O
the	O
USB	B-Protocol
3.0	O
driver	O
Sage	O
Sharp	O
asked	O
Torvalds	O
to	O
address	O
the	O
abusive	O
commentary	O
in	O
the	O
kernel	B-Operating_System
development	O
community	O
.	O
</s>
<s>
In	O
2014	O
,	O
Sharp	O
backed	O
out	O
of	O
Linux	B-Application
kernel	B-Operating_System
development	O
,	O
saying	O
that	O
"	O
The	O
focus	O
on	O
technical	O
excellence	O
,	O
in	O
combination	O
with	O
overloaded	O
maintainers	O
,	O
and	O
people	O
with	O
different	O
cultural	O
and	O
social	O
norms	O
,	O
means	O
that	O
Linux	B-Application
kernel	B-Operating_System
maintainers	O
are	O
often	O
blunt	O
,	O
rude	O
,	O
or	O
brutal	O
to	O
get	O
their	O
job	O
done	O
"	O
.	O
</s>
<s>
At	B-Operating_System
the	O
linux.conf.au	O
(	O
LCA	O
)	O
conference	O
in	O
2018	O
,	O
developers	O
expressed	O
the	O
view	O
that	O
the	O
culture	O
of	O
the	O
community	O
has	O
gotten	O
much	O
better	O
in	O
the	O
past	O
few	O
years	O
.	O
</s>
<s>
Daniel	O
Vetter	O
,	O
the	O
maintainer	O
of	O
the	O
Intel	O
drm/i915	O
graphics	O
kernel	B-Application
driver	I-Application
,	O
commented	O
that	O
the	O
"	O
rather	O
violent	O
language	O
and	O
discussion	O
"	O
in	O
the	O
kernel	B-Operating_System
community	O
has	O
decreased	O
or	O
disappeared	O
.	O
</s>
<s>
Laurent	O
Pinchart	O
asked	O
developers	O
for	O
feedback	O
on	O
their	O
experience	O
with	O
the	O
kernel	B-Operating_System
community	O
at	B-Operating_System
the	O
2017	O
Embedded	B-Application
Linux	I-Application
Conference	O
Europe	O
.	O
</s>
<s>
The	O
issues	O
brought	O
up	O
were	O
discussed	O
a	O
few	O
days	O
later	O
at	B-Operating_System
the	O
Maintainers	O
Summit	B-Device
.	O
</s>
<s>
Concerns	O
over	O
the	O
lack	O
of	O
consistency	O
in	O
how	O
maintainers	O
responded	O
to	O
patches	O
submitted	O
by	O
developers	O
were	O
echoed	O
by	O
Shuah	O
Khan	O
,	O
the	O
maintainer	O
of	O
the	O
kernel	B-Operating_System
self-test	O
framework	O
.	O
</s>
<s>
Torvalds	O
contended	O
that	O
there	O
would	O
never	O
be	O
consistency	O
in	O
the	O
handling	O
of	O
patches	O
because	O
different	O
kernel	B-Operating_System
subsystems	O
have	O
,	O
over	O
time	O
,	O
adopted	O
different	O
development	O
processes	B-Operating_System
.	O
</s>
<s>
Therefore	O
,	O
it	O
was	O
agreed	O
upon	O
that	O
each	O
kernel	B-Operating_System
subsystem	O
maintainer	O
would	O
document	O
the	O
rules	O
for	O
patch	O
acceptance	O
.	O
</s>
<s>
The	O
Git	B-Application
tree	O
of	O
Linus	O
Torvalds	O
that	O
contains	O
the	O
Linux	B-Application
kernel	B-Operating_System
is	O
referred	O
to	O
as	O
mainline	B-Application
Linux	I-Application
.	O
</s>
<s>
Every	O
stable	O
kernel	B-Operating_System
release	O
originates	O
from	O
the	O
mainline	O
tree	O
,	O
and	O
is	O
frequently	O
published	O
on	O
kernel.org	B-Application
.	O
</s>
<s>
Mainline	B-Application
Linux	I-Application
has	O
only	O
solid	O
support	O
for	O
a	O
small	O
subset	O
of	O
the	O
many	O
devices	O
that	O
run	O
Linux	B-Application
.	O
</s>
<s>
Non-mainline	O
support	O
is	O
provided	O
by	O
independent	O
projects	O
,	O
such	O
as	O
Yocto	B-Application
or	O
Linaro	O
,	O
but	O
in	O
many	O
cases	O
the	O
kernel	B-Operating_System
from	O
the	O
device	O
vendor	O
is	O
needed	O
.	O
</s>
<s>
Using	O
a	O
vendor	O
kernel	B-Operating_System
likely	O
requires	O
a	O
board	B-Operating_System
support	I-Operating_System
package	I-Operating_System
.	O
</s>
<s>
Maintaining	O
a	O
kernel	B-Operating_System
tree	O
outside	O
of	O
mainline	B-Application
Linux	I-Application
has	O
proven	O
to	O
be	O
difficult	O
.	O
</s>
<s>
Mainlining	O
refers	O
to	O
the	O
effort	O
of	O
adding	O
support	O
for	O
a	O
device	O
to	O
the	O
mainline	O
kernel	B-Operating_System
,	O
while	O
there	O
was	O
formerly	O
only	O
support	O
in	O
a	O
fork	B-Application
or	O
no	O
support	O
at	B-Operating_System
all	O
.	O
</s>
<s>
This	O
usually	O
includes	O
adding	O
drivers	O
or	O
device	B-Operating_System
tree	I-Operating_System
files	O
.	O
</s>
<s>
The	O
maintainer	O
of	O
the	O
stable	O
branch	O
,	O
Greg	O
Kroah-Hartman	O
,	O
has	O
applied	O
the	O
term	O
Linux-like	O
to	O
downstream	B-General_Concept
kernel	B-Operating_System
forks	B-Application
by	O
vendors	O
that	O
add	O
millions	O
of	O
lines	O
of	O
code	O
to	O
the	O
mainline	O
kernel	B-Operating_System
.	O
</s>
<s>
In	O
2019	O
,	O
Google	B-Application
stated	O
that	O
they	O
wanted	O
to	O
use	O
the	O
mainline	B-Application
Linux	I-Application
kernel	B-Operating_System
in	O
Android	B-Application
so	O
the	O
number	O
of	O
kernel	B-Operating_System
forks	B-Application
would	O
be	O
reduced	O
.	O
</s>
<s>
The	O
term	O
Linux-like	O
has	O
also	O
been	O
applied	O
to	O
the	O
Embeddable	B-Application
Linux	I-Application
Kernel	I-Application
Subset	I-Application
,	O
which	O
does	O
not	O
include	O
the	O
full	O
mainline	B-Application
Linux	I-Application
kernel	B-Operating_System
but	O
a	O
small	O
modified	O
subset	O
of	O
the	O
code	O
.	O
</s>
<s>
There	O
are	O
certain	O
communities	O
that	O
develop	O
kernels	B-Operating_System
based	O
on	O
the	O
official	O
Linux	B-Application
.	O
</s>
<s>
Some	O
interesting	O
bits	O
of	O
code	O
from	O
these	O
forks	B-Application
that	O
include	O
Linux-libre	B-Application
,	O
Compute	B-Application
Node	I-Application
Linux	I-Application
,	O
INK	B-Application
,	O
L4Linux	B-Application
,	O
RTLinux	B-Application
,	O
and	O
User-Mode	B-Application
Linux	I-Application
(	O
UML	O
)	O
have	O
been	O
merged	O
into	O
the	O
mainline	O
.	O
</s>
<s>
Some	O
operating	O
systems	O
developed	O
for	O
mobile	O
phones	O
initially	O
used	O
heavily	O
modified	O
versions	O
of	O
Linux	B-Application
,	O
including	O
Google	B-Application
Android	I-Application
,	O
Firefox	B-Operating_System
OS	I-Operating_System
,	O
HP	B-Operating_System
webOS	I-Operating_System
,	O
Nokia	B-Operating_System
Maemo	I-Operating_System
and	O
Jolla	O
Sailfish	B-Operating_System
OS	I-Operating_System
.	O
</s>
<s>
In	O
2010	O
,	O
the	O
Linux	B-Application
community	I-Application
criticised	O
Google	B-Application
for	O
effectively	O
starting	O
its	O
own	O
kernel	B-Operating_System
tree	O
:	O
</s>
<s>
Today	O
Android	B-Application
uses	O
a	O
customized	O
Linux	B-Application
where	O
major	O
changes	O
are	O
implemented	O
in	O
device	B-Application
drivers	I-Application
,	O
but	O
some	O
changes	O
to	O
the	O
core	O
kernel	B-Operating_System
code	O
is	O
required	O
.	O
</s>
<s>
Android	B-Application
developers	O
also	O
submit	O
patches	O
to	O
the	O
official	O
Linux	B-Application
that	O
finally	O
can	O
boot	O
the	O
Android	B-Application
operating	I-Application
system	I-Application
.	O
</s>
<s>
For	O
example	O
,	O
a	O
Nexus	B-Device
7	I-Device
can	O
boot	O
and	O
run	O
the	O
mainline	B-Application
Linux	I-Application
.	O
</s>
<s>
At	B-Operating_System
a	O
2001	O
presentation	O
at	B-Operating_System
the	O
Computer	O
History	O
Museum	O
,	O
Linus	O
Torvalds	O
had	O
this	O
to	O
say	O
in	O
response	O
to	O
a	O
question	O
about	O
distributions	O
of	O
Linux	B-Application
using	O
precisely	O
the	O
same	O
kernel	B-Operating_System
sources	O
or	O
not	O
:	O
</s>
<s>
There	O
have	O
been	O
several	O
notable	O
conflicts	O
among	O
Linux	B-Application
kernel	B-Operating_System
developers	O
.	O
</s>
<s>
In	O
July	O
2007	O
,	O
Con	O
Kolivas	O
announced	O
that	O
he	O
would	O
cease	O
developing	O
for	O
the	O
Linux	B-Application
kernel	B-Operating_System
.	O
</s>
<s>
In	O
July	O
2009	O
,	O
Alan	O
Cox	O
quit	O
his	O
role	O
as	O
the	O
TTY	B-Device
layer	O
maintainer	O
after	O
disagreement	O
with	O
Linus	O
Torvalds	O
.	O
</s>
<s>
In	O
December	O
2010	O
,	O
there	O
was	O
a	O
discussion	O
between	O
Linux	B-Application
SCSI	B-Architecture
maintainer	O
James	O
Bottomley	O
and	O
SCST	O
maintainer	O
Vladislav	O
Bolkhovitin	O
about	O
which	O
SCSI	B-Architecture
target	O
stack	O
should	O
be	O
included	O
in	O
the	O
Linux	B-Application
kernel	B-Operating_System
.	O
</s>
<s>
This	O
made	O
some	O
Linux	B-Application
users	O
upset	O
.	O
</s>
<s>
In	O
April	O
2014	O
,	O
Torvalds	O
banned	O
Kay	O
Sievers	O
from	O
submitting	O
patches	O
to	O
the	O
Linux	B-Application
kernel	B-Operating_System
for	O
failing	O
to	O
deal	O
with	O
bugs	B-Error_Name
that	O
caused	O
systemd	B-Application
to	O
negatively	O
interact	O
with	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
In	O
October	O
2014	O
,	O
Lennart	O
Poettering	O
accused	O
Torvalds	O
of	O
tolerating	O
the	O
rough	O
discussion	O
style	O
on	O
Linux	B-Application
kernel	B-Operating_System
related	O
mailing	O
lists	O
and	O
of	O
being	O
a	O
bad	O
role	O
model	O
.	O
</s>
<s>
In	O
March	O
2015	O
,	O
Christoph	O
Hellwig	O
filed	O
a	O
lawsuit	O
against	O
VMware	O
for	O
infringement	O
of	O
the	O
copyright	O
on	O
the	O
Linux	B-Application
kernel	B-Operating_System
.	O
</s>
<s>
In	O
April	O
2021	O
,	O
a	O
team	O
from	O
the	O
University	O
of	O
Minnesota	O
was	O
found	O
to	O
be	O
submitting	O
"	O
bad	O
faith	O
"	O
patches	O
to	O
the	O
kernel	B-Operating_System
as	O
part	O
of	O
their	O
research	O
.	O
</s>
<s>
Prominent	O
Linux	B-Application
kernel	B-Operating_System
developers	O
have	O
been	O
aware	O
of	O
the	O
importance	O
of	O
avoiding	O
conflicts	O
between	O
developers	O
.	O
</s>
<s>
For	O
a	O
long	O
time	O
there	O
was	O
no	O
code	O
of	O
conduct	O
for	O
kernel	B-Operating_System
developers	O
due	O
to	O
opposition	O
by	O
Linus	O
Torvalds	O
.	O
</s>
<s>
However	O
,	O
a	O
Linux	B-Application
Kernel	B-Operating_System
Code	O
of	O
Conflict	O
was	O
introduced	O
on	O
8	O
March	O
2015	O
.	O
</s>
<s>
This	O
coincided	O
with	O
a	O
public	O
apology	O
by	O
Torvalds	O
and	O
a	O
brief	O
break	O
from	O
kernel	B-Operating_System
development	O
.	O
</s>
<s>
,	O
the	O
5.11	O
release	O
of	O
the	O
Linux	B-Application
kernel	B-Operating_System
had	O
around	O
30.34	O
million	O
lines	O
of	O
code	O
.	O
</s>
<s>
Roughly	O
14%	O
of	O
the	O
code	O
is	O
part	O
of	O
the	O
"	O
core	O
"	O
(	O
arch	O
,	O
kernel	B-Operating_System
and	O
mm	O
directories	O
)	O
,	O
while	O
60%	O
is	O
drivers	O
.	O
</s>
<s>
The	O
cost	O
to	O
redevelop	O
version	O
2.6.0	O
of	O
the	O
Linux	B-Application
kernel	B-Operating_System
in	O
a	O
traditional	O
proprietary	B-Application
development	O
setting	O
has	O
been	O
estimated	O
to	O
be	O
US$612	O
million	O
( €467M	O
,	O
£394M	O
)	O
in	O
2004	O
prices	O
using	O
the	O
COCOMO	O
person-month	O
estimation	O
model	O
.	O
</s>
<s>
In	O
2006	O
,	O
a	O
study	O
funded	O
by	O
the	O
European	O
Union	O
put	O
the	O
redevelopment	O
cost	O
of	O
kernel	B-Operating_System
version	O
2.6.8	O
higher	O
,	O
at	B-Operating_System
€882M	O
(	O
$1.14	O
bn	O
,	O
£744M	O
)	O
.	O
</s>
<s>
Using	O
David	O
A	O
.	O
Wheeler	O
's	O
methodology	O
,	O
they	O
estimated	O
redevelopment	O
of	O
the	O
2.6.25	O
kernel	B-Operating_System
now	O
costs	O
$1.3	O
bn	O
(	O
part	O
of	O
a	O
total	O
$10.8	O
bn	O
to	O
redevelop	O
Fedora	O
9	O
)	O
.	O
</s>
<s>
Again	O
,	O
Garcia-Garcia	O
and	O
Alonso	O
de	O
Magdaleno	O
from	O
University	O
of	O
Oviedo	O
(	O
Spain	O
)	O
estimate	O
that	O
the	O
value	O
annually	O
added	O
to	O
kernel	B-Operating_System
was	O
about	O
€100M	O
between	O
2005	O
and	O
2007	O
and	O
€225M	O
in	O
2008	O
,	O
it	O
would	O
cost	O
also	O
more	O
than	O
€1bn	O
(	O
about	O
$1.4	O
bn	O
as	O
of	O
February	O
2010	O
)	O
to	O
develop	O
in	O
the	O
European	O
Union	O
.	O
</s>
<s>
,	O
using	O
then-current	O
LOC	O
(	O
lines	O
of	O
code	O
)	O
of	O
a	O
2.6.x	O
Linux	B-Application
kernel	B-Operating_System
and	O
wage	O
numbers	O
with	O
David	O
A	O
.	O
Wheeler	O
's	O
calculations	O
it	O
would	O
cost	O
approximately	O
$3bn	O
(	O
about	O
€2.2	O
bn	O
)	O
to	O
redevelop	O
the	O
Linux	B-Application
kernel	B-Operating_System
as	O
it	O
keeps	O
getting	O
bigger	O
.	O
</s>
<s>
An	O
updated	O
calculation	O
,	O
using	O
then-current	O
20,088,609	O
LOC	O
(	O
lines	O
of	O
code	O
)	O
for	O
the	O
4.14.14	O
Linux	B-Application
kernel	B-Operating_System
and	O
the	O
current	O
US	O
national	O
average	O
programmer	O
salary	O
of	O
$	O
75,506	O
show	O
it	O
would	O
cost	O
approximately	O
$	O
14,725,449,000	O
dollars	O
( £	O
11,191,341,000	O
)	O
to	O
rewrite	O
the	O
existing	O
code	O
.	O
</s>
<s>
The	O
latest	O
kernel	B-Operating_System
version	O
and	O
older	O
kernel	B-Operating_System
versions	O
are	O
maintained	O
separately	O
.	O
</s>
<s>
Most	O
latest	O
kernel	B-Operating_System
releases	O
were	O
supervised	O
by	O
Linus	O
Torvalds	O
.	O
</s>
<s>
The	O
Linux	B-Application
kernel	B-Operating_System
developer	O
community	O
maintains	O
a	O
stable	O
kernel	B-Operating_System
by	O
applying	O
fixes	O
for	O
software	B-Error_Name
bugs	I-Error_Name
that	O
have	O
been	O
discovered	O
during	O
the	O
development	O
of	O
the	O
subsequent	O
stable	O
kernel	B-Operating_System
.	O
</s>
<s>
Therefore	O
,	O
www.kernel.org	O
will	O
always	O
list	O
two	O
stable	O
kernels	B-Operating_System
.	O
</s>
<s>
The	O
next	O
stable	O
Linux	B-Application
kernel	B-Operating_System
is	O
now	O
released	O
only	O
8	O
to	O
12	O
weeks	O
later	O
.	O
</s>
<s>
Therefore	O
,	O
the	O
Linux	B-Application
kernel	B-Operating_System
maintainers	O
have	O
designated	O
some	O
stable	O
kernel	B-Operating_System
releases	O
as	O
longterm	O
,	O
these	O
long-term	O
support	O
Linux	B-Application
kernels	B-Operating_System
are	O
updated	O
with	O
bug	B-Error_Name
fixes	I-Error_Name
for	O
two	O
or	O
more	O
years	O
.	O
</s>
<s>
,	O
there	O
are	O
six	O
longterm	O
Linux	B-Application
kernels	B-Operating_System
:	O
5.15.23	O
,	O
5.10.100	O
,	O
5.4.179	O
,	O
4.19.229	O
,	O
4.14.266	O
,	O
and	O
4.9.301	O
.	O
</s>
<s>
The	O
full	O
list	O
of	O
releases	O
is	O
at	B-Operating_System
Linux	B-Application
kernel	I-Application
version	I-Application
history	I-Application
.	O
</s>
<s>
Most	O
Linux	B-Application
users	O
run	O
a	O
kernel	B-Operating_System
supplied	O
by	O
their	O
Linux	B-Application
distribution	I-Application
.	O
</s>
<s>
Some	O
distributions	O
ship	O
the	O
"	O
vanilla	O
"	O
or	O
"	O
stable	O
"	O
kernels	B-Operating_System
.	O
</s>
<s>
However	O
,	O
several	O
Linux	B-Application
distribution	I-Application
vendors	O
(	O
such	O
as	O
Red	O
Hat	O
and	O
Debian	O
)	O
maintain	O
another	O
set	O
of	O
Linux	B-Application
kernel	B-Operating_System
branches	O
which	O
are	O
integrated	O
into	O
their	O
products	O
.	O
</s>
<s>
These	O
are	O
usually	O
updated	O
at	B-Operating_System
a	O
slower	O
pace	O
compared	O
to	O
the	O
"	O
vanilla	O
"	O
branch	O
,	O
and	O
they	O
usually	O
include	O
all	O
fixes	O
from	O
the	O
relevant	O
"	O
stable	O
"	O
branch	O
,	O
but	O
at	B-Operating_System
the	O
same	O
time	O
they	O
can	O
also	O
add	O
support	O
for	O
drivers	O
or	O
features	O
which	O
had	O
not	O
been	O
released	O
in	O
the	O
"	O
vanilla	O
"	O
version	O
the	O
distribution	O
vendor	O
started	O
basing	O
their	O
branch	O
from	O
.	O
</s>
<s>
Initially	O
,	O
Torvalds	O
released	O
Linux	B-Application
under	O
a	O
license	O
which	O
forbade	O
any	O
commercial	O
use	O
.	O
</s>
<s>
This	O
was	O
changed	O
in	O
version	O
0.12	O
by	O
a	O
switch	O
to	O
the	O
GNU	B-License
General	I-License
Public	I-License
License	I-License
version	O
2	O
(	O
GPLv2	O
)	O
.	O
</s>
<s>
This	O
license	O
allows	O
distribution	O
and	O
sale	O
of	O
possibly	O
modified	O
and	O
unmodified	O
versions	O
of	O
Linux	B-Application
but	O
requires	O
that	O
all	O
those	O
copies	O
be	O
released	O
under	O
the	O
same	O
license	O
and	O
be	O
accompanied	O
by	O
-	O
or	O
that	O
,	O
on	O
request	O
,	O
free	O
access	O
is	O
given	O
to	O
-	O
the	O
complete	O
corresponding	O
source	O
code	O
.	O
</s>
<s>
Torvalds	O
has	O
described	O
licensing	O
Linux	B-Application
under	O
the	O
GPLv2	O
as	O
the	O
"	O
best	O
thing	O
I	O
ever	O
did	O
"	O
.	O
</s>
<s>
The	O
Linux	B-Application
kernel	B-Operating_System
is	O
licensed	O
explicitly	O
under	O
GNU	B-License
General	I-License
Public	I-License
License	I-License
version	I-License
2	I-License
only	I-License
(	O
GPL-2.0-only	B-License
)	O
with	O
an	O
explicit	O
syscall	B-Operating_System
exception	O
(	O
Linux-syscall-note	O
)	O
,	O
without	O
offering	O
the	O
licensee	O
the	O
option	O
to	O
choose	O
any	O
later	O
version	O
,	O
which	O
is	O
a	O
common	O
GPL	B-License
extension	O
.	O
</s>
<s>
Contributed	O
code	O
must	O
be	O
available	O
under	O
GPL-compatible	O
license	O
.	O
</s>
<s>
There	O
was	O
considerable	O
debate	O
about	O
how	O
easily	O
the	O
license	O
could	O
be	O
changed	O
to	O
use	O
later	O
GPL	B-License
versions	O
(	O
including	O
version	O
3	O
)	O
,	O
and	O
whether	O
this	O
change	O
is	O
even	O
desirable	O
.	O
</s>
<s>
However	O
,	O
the	O
terms	O
of	O
the	O
GPL	B-License
state	O
that	O
if	O
no	O
version	O
is	O
specified	O
,	O
then	O
any	O
version	O
may	O
be	O
used	O
,	O
and	O
Alan	O
Cox	O
pointed	O
out	O
that	O
very	O
few	O
other	O
Linux	B-Application
contributors	O
had	O
specified	O
a	O
particular	O
version	O
of	O
the	O
GPL	B-License
.	O
</s>
<s>
In	O
September	O
2006	O
,	O
a	O
survey	O
of	O
29	O
key	O
kernel	B-Operating_System
programmers	O
indicated	O
that	O
28	O
preferred	O
GPLv2	O
to	O
the	O
then-current	O
GPLv3	O
draft	O
.	O
</s>
<s>
This	O
group	O
of	O
high-profile	O
kernel	B-Operating_System
developers	O
,	O
including	O
Torvalds	O
,	O
Greg	O
Kroah-Hartman	O
and	O
Andrew	O
Morton	O
,	O
commented	O
on	O
mass	O
media	O
about	O
their	O
objections	O
to	O
the	O
GPLv3	O
.	O
</s>
<s>
They	O
referred	O
to	O
clauses	O
regarding	O
DRM/tivoization	O
,	O
patents	O
,	O
"	O
additional	O
restrictions	O
"	O
and	O
warned	O
a	O
Balkanisation	O
of	O
the	O
"	O
Open	O
Source	O
Universe	O
"	O
by	O
the	O
GPLv3	O
.	O
</s>
<s>
Linus	O
Torvalds	O
,	O
who	O
decided	O
not	O
to	O
adopt	O
the	O
GPLv3	O
for	O
the	O
Linux	B-Application
kernel	B-Operating_System
,	O
reiterated	O
his	O
criticism	O
even	O
years	O
later	O
.	O
</s>
<s>
It	O
is	O
debated	O
whether	O
some	O
loadable	B-Application
kernel	I-Application
modules	I-Application
(	O
LKMs	O
)	O
are	O
to	O
be	O
considered	O
derivative	O
works	O
under	O
copyright	O
law	O
,	O
and	O
thereby	O
whether	O
or	O
not	O
they	O
fall	O
under	O
the	O
terms	O
of	O
the	O
GPL	B-License
.	O
</s>
<s>
In	O
accordance	O
with	O
the	O
license	O
rules	O
,	O
LKMs	O
using	O
only	O
a	O
public	O
subset	O
of	O
the	O
kernel	B-Operating_System
interfaces	O
are	O
non-derived	O
works	O
,	O
thus	O
Linux	B-Application
gives	O
system	O
administrators	O
the	O
mechanisms	O
to	O
load	O
out-of-tree	O
binary	O
objects	O
into	O
the	O
kernel	B-Operating_System
address	O
space	O
.	O
</s>
<s>
There	O
are	O
some	O
out-of-tree	O
loadable	O
modules	B-Application
that	O
make	B-Application
legitimate	O
use	O
of	O
the	O
dma_buf	O
kernel	B-Operating_System
feature	O
.	O
</s>
<s>
GPL	B-License
compliant	O
code	O
can	O
certainly	O
use	O
it	O
.	O
</s>
<s>
However	O
,	O
a	O
different	O
possible	O
use	O
case	O
would	O
be	O
Nvidia	B-Application
Optimus	I-Application
that	O
pairs	O
a	O
fast	O
GPU	B-Architecture
with	O
an	O
Intel	O
integrated	O
GPU	B-Architecture
,	O
where	O
the	O
Nvidia	O
GPU	B-Architecture
writes	O
into	O
the	O
Intel	O
framebuffer	O
when	O
it	O
is	O
active	O
.	O
</s>
<s>
But	O
,	O
Nvidia	O
cannot	O
use	O
this	O
infrastructure	O
because	O
it	O
necessitates	O
bypassing	O
a	O
rule	O
that	O
can	O
only	O
be	O
used	O
by	O
LKMs	O
that	O
are	O
also	O
GPL	B-License
.	O
</s>
<s>
Alan	O
Cox	O
replied	O
on	O
LKML	B-Application
,	O
rejecting	O
a	O
request	O
from	O
one	O
of	O
their	O
engineers	O
to	O
remove	O
this	O
technical	O
enforcement	O
from	O
the	O
API	B-Application
.	O
</s>
<s>
Torvalds	O
clearly	O
stated	O
on	O
the	O
LKML	B-Application
that	O
"[I]	O
claim	O
that	O
binary-only	O
kernel	B-Application
modules	I-Application
ARE	O
derivative	O
"	O
by	O
default	O
"'"	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
Torvalds	O
has	O
also	O
said	O
that	O
"[one]	O
gray	O
area	O
in	O
particular	O
is	O
something	O
like	O
a	O
driver	O
that	O
was	O
originally	O
written	O
for	O
another	O
operating	O
system	O
(	O
i.e.	O
,	O
clearly	O
not	O
a	O
derived	O
work	O
of	O
Linux	B-Application
in	O
origin	O
)	O
.	O
</s>
<s>
THAT	O
is	O
a	O
gray	O
area	O
,	O
and	O
_that_	O
is	O
the	O
area	O
where	O
I	O
personally	O
believe	O
that	O
some	O
modules	B-Application
may	O
be	O
considered	O
to	O
not	O
be	O
derived	O
works	O
simply	O
because	O
they	O
were	O
n't	O
designed	O
for	O
Linux	B-Application
and	O
do	O
n't	O
depend	O
on	O
any	O
special	O
Linux	B-Application
behaviour	O
"	O
.	O
</s>
<s>
Proprietary	B-Application
graphics	O
drivers	O
,	O
in	O
particular	O
,	O
are	O
heavily	O
discussed	O
.	O
</s>
<s>
Whenever	O
proprietary	B-Application
modules	B-Application
are	O
loaded	O
into	O
Linux	B-Application
,	O
the	O
kernel	B-Operating_System
marks	O
itself	O
as	O
being	O
"	O
tainted	O
"	O
,	O
and	O
therefore	O
bug	B-Error_Name
reports	O
from	O
tainted	O
kernels	B-Operating_System
will	O
often	O
be	O
ignored	O
by	O
developers	O
.	O
</s>
<s>
The	O
official	O
kernel	B-Operating_System
,	O
that	O
is	O
the	O
Linus	O
git	B-Application
branch	O
at	B-Operating_System
the	O
kernel.org	B-Application
repository	O
,	O
contains	O
binary	O
blobs	O
released	O
under	O
the	O
terms	O
of	O
the	O
GNU	B-Operating_System
GPLv2	O
license	O
.	O
</s>
<s>
Linux	B-Application
can	O
also	O
search	O
filesystems	B-Application
to	O
locate	O
binary	O
blobs	O
,	O
proprietary	B-Application
firmware	O
,	O
drivers	O
,	O
or	O
other	O
executable	O
modules	B-Application
,	O
then	O
it	O
can	O
load	O
and	O
link	O
them	O
into	O
kernel	B-Operating_System
space	I-Operating_System
.	O
</s>
<s>
When	O
it	O
is	O
needed	O
(	O
e.g.	O
,	O
for	O
accessing	O
boot	O
devices	O
or	O
for	O
speed	O
)	O
firmware	O
can	O
be	O
built-in	O
to	O
the	O
kernel	B-Operating_System
,	O
this	O
means	O
building	O
the	O
firmware	O
into	O
vmlinux	B-Application
;	O
however	O
this	O
is	O
not	O
always	O
a	O
viable	O
option	O
for	O
technical	O
or	O
legal	O
issues	O
(	O
e.g.	O
,	O
it	O
is	O
not	O
permitted	O
to	O
do	O
this	O
with	O
firmware	O
that	O
is	O
non-GPL	O
compatible	O
,	O
although	O
this	O
is	O
quite	O
common	O
nonetheless	O
)	O
.	O
</s>
<s>
Linux	B-Application
is	O
a	O
registered	O
trademark	O
of	O
Linus	O
Torvalds	O
in	O
the	O
United	O
States	O
,	O
the	O
European	O
Union	O
,	O
and	O
some	O
other	O
countries	O
.	O
</s>
<s>
A	O
legal	O
battle	O
over	O
the	O
trademark	O
began	O
in	O
1996	O
,	O
when	O
William	O
Della	O
Croce	O
,	O
a	O
lawyer	O
who	O
was	O
never	O
involved	O
in	O
the	O
development	O
of	O
Linux	B-Application
,	O
started	O
requesting	O
licensing	O
fees	O
for	O
the	O
use	O
of	O
the	O
word	O
Linux	B-Application
.	O
</s>
