<s>
In	O
operating	B-General_Concept
systems	I-General_Concept
,	O
a	O
giant	B-Operating_System
lock	I-Operating_System
,	O
also	O
known	O
as	O
a	O
big-lock	O
or	O
kernel-lock	O
,	O
is	O
a	O
lock	B-Operating_System
that	O
may	O
be	O
used	O
in	O
the	O
kernel	B-Operating_System
to	O
provide	O
concurrency	B-Operating_System
control	I-Operating_System
required	O
by	O
symmetric	B-Operating_System
multiprocessing	I-Operating_System
(	O
SMP	O
)	O
systems	O
.	O
</s>
<s>
A	O
giant	B-Operating_System
lock	I-Operating_System
is	O
a	O
solitary	O
global	O
lock	B-Operating_System
that	O
is	O
held	O
whenever	O
a	O
thread	B-Operating_System
enters	O
kernel	B-Operating_System
space	I-Operating_System
and	O
released	O
when	O
the	O
thread	B-Operating_System
returns	O
to	O
user	B-Operating_System
space	I-Operating_System
;	O
a	O
system	B-Operating_System
call	I-Operating_System
is	O
the	O
archetypal	O
example	O
.	O
</s>
<s>
In	O
this	O
model	O
,	O
threads	B-Operating_System
in	O
user	B-Operating_System
space	I-Operating_System
can	O
run	O
concurrently	O
on	O
any	O
available	O
processors	B-Architecture
or	O
processor	B-Architecture
cores	I-Architecture
,	O
but	O
no	O
more	O
than	O
one	O
thread	B-Operating_System
can	O
run	O
in	O
kernel	B-Operating_System
space	I-Operating_System
;	O
any	O
other	O
threads	B-Operating_System
that	O
try	O
to	O
enter	O
kernel	B-Operating_System
space	I-Operating_System
are	O
forced	O
to	O
wait	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
the	O
giant	B-Operating_System
lock	I-Operating_System
eliminates	O
all	O
concurrency	B-Operating_System
in	O
kernel	B-Operating_System
space	I-Operating_System
.	O
</s>
<s>
By	O
isolating	O
the	O
kernel	B-Operating_System
from	O
concurrency	B-Operating_System
,	O
many	O
parts	O
of	O
the	O
kernel	B-Operating_System
no	O
longer	O
need	O
to	O
be	O
modified	O
to	O
support	O
SMP	O
.	O
</s>
<s>
However	O
,	O
as	O
in	O
giant-lock	O
SMP	O
systems	O
only	O
one	O
processor	O
can	O
run	O
the	O
kernel	B-Operating_System
code	O
at	O
a	O
time	O
,	O
performance	O
for	O
applications	O
spending	O
significant	O
amounts	O
of	O
time	O
in	O
the	O
kernel	B-Operating_System
is	O
not	O
much	O
improved	O
.	O
</s>
<s>
Accordingly	O
,	O
the	O
giant-lock	O
approach	O
is	O
commonly	O
seen	O
as	O
a	O
preliminary	O
means	O
of	O
bringing	O
SMP	O
support	O
to	O
an	O
operating	B-General_Concept
system	I-General_Concept
,	O
yielding	O
benefits	O
only	O
in	O
user	B-Operating_System
space	I-Operating_System
.	O
</s>
<s>
Most	O
modern	O
operating	B-General_Concept
systems	I-General_Concept
use	O
a	O
fine-grained	O
locking	B-Operating_System
approach	O
.	O
</s>
<s>
The	O
Linux	B-Operating_System
kernel	I-Operating_System
had	O
a	O
big	B-Operating_System
kernel	I-Operating_System
lock	I-Operating_System
(	O
BKL	O
)	O
since	O
the	O
introduction	O
of	O
SMP	O
,	O
until	O
Arnd	O
Bergmann	O
removed	O
it	O
in	O
2011	O
in	O
kernel	B-Operating_System
version	O
2.6.39	O
,	O
with	O
the	O
remaining	O
uses	O
of	O
the	O
big	O
lock	B-Operating_System
removed	O
or	O
replaced	O
by	O
finer-grained	O
locking	B-Operating_System
.	O
</s>
<s>
Linux	B-Application
distributions	I-Application
at	O
or	O
above	O
CentOS	O
7	O
,	O
Debian	O
7	O
(	O
Wheezy	O
)	O
and	O
Ubuntu	O
11.10	O
are	O
therefore	O
not	O
using	O
BKL	O
.	O
</s>
<s>
,	O
Linux	B-Operating_System
kernel	I-Operating_System
still	O
has	O
console_lock	O
which	O
is	O
sometimes	O
referred	O
as	O
BKL	O
and	O
its	O
removal	O
is	O
in	O
progress	O
.	O
</s>
<s>
,	O
OpenBSD	B-Operating_System
and	O
NetBSD	B-Device
are	O
still	O
using	O
the	O
spl	B-Operating_System
(	O
Unix	O
)	O
family	O
of	O
primitives	O
to	O
facilitate	O
synchronisation	O
of	O
critical	O
sections	O
within	O
the	O
kernel	B-Operating_System
,	O
meaning	O
that	O
many	O
system	B-Operating_System
calls	I-Operating_System
may	O
inhibit	O
SMP	O
capabilities	O
of	O
the	O
system	O
,	O
and	O
,	O
according	O
to	O
Matthew	O
Dillon	O
,	O
the	O
SMP	O
capabilities	O
of	O
these	O
two	O
systems	O
cannot	O
be	O
considered	O
modern	O
.	O
</s>
<s>
FreeBSD	B-Operating_System
still	O
has	O
support	O
for	O
the	O
Giant	O
mutex	B-Operating_System
,	O
which	O
provides	O
semantics	O
akin	O
to	O
the	O
old	O
spl	B-Operating_System
interface	O
,	O
but	O
performance-critical	O
core	O
components	O
have	O
long	O
been	O
converted	O
to	O
use	O
finer-grained	O
locking	B-Operating_System
.	O
</s>
<s>
It	O
is	O
claimed	O
by	O
Matthew	O
Dillon	O
that	O
out	O
of	O
the	O
open-source	B-Application
software	I-Application
general-purpose	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
,	O
only	O
Linux	B-Application
,	O
DragonFly	B-Application
BSD	I-Application
and	O
FreeBSD	B-Operating_System
have	O
modern	O
SMP	O
support	O
,	O
with	O
OpenBSD	B-Operating_System
and	O
NetBSD	B-Device
falling	O
behind	O
.	O
</s>
<s>
The	O
NetBSD	B-Device
Foundation	O
views	O
modern	O
SMP	O
support	O
as	O
vital	O
to	O
the	O
direction	O
of	O
The	O
NetBSD	B-Device
Project	O
,	O
and	O
has	O
offered	O
grants	O
to	O
developers	O
willing	O
to	O
work	O
on	O
SMP	O
improvements	O
;	O
NPF	B-Protocol
(	O
firewall	O
)	O
was	O
one	O
of	O
the	O
projects	O
that	O
arose	O
as	O
a	O
result	O
of	O
these	O
financial	O
incentives	O
,	O
but	O
further	O
improvements	O
to	O
the	O
core	O
networking	O
stack	O
may	O
still	O
be	O
necessary	O
.	O
</s>
