path: root/Documentation/spinlocks.txt
diff options
authorMuthu Kumar <>2011-07-11 11:04:58 -0700
committerLinus Torvalds <>2011-07-11 12:45:04 -0700
commit05801817845b308e1cf0fb8e2700b15dab79afc5 (patch)
treed2049ad193b44c387446f295ff1abe0b8647fa0e /Documentation/spinlocks.txt
parente3bbfa78bab125f58b831b5f7f45b5a305091d72 (diff)
Documentation/spinlocks.txt: Remove reference to sti()/cli()
Since we removed sti()/cli() and related, how about removing it from Documentation/spinlocks.txt? Signed-off-by: Muthukumar R <> Signed-off-by: Linus Torvalds <>
Diffstat (limited to 'Documentation/spinlocks.txt')
1 files changed, 7 insertions, 38 deletions
diff --git a/Documentation/spinlocks.txt b/Documentation/spinlocks.txt
index 2e3c64b1a6a5..9dbe885ecd8d 100644
--- a/Documentation/spinlocks.txt
+++ b/Documentation/spinlocks.txt
@@ -13,18 +13,8 @@ static DEFINE_SPINLOCK(xxx_lock);
The above is always safe. It will disable interrupts _locally_, but the
spinlock itself will guarantee the global lock, so it will guarantee that
there is only one thread-of-control within the region(s) protected by that
-lock. This works well even under UP. The above sequence under UP
-essentially is just the same as doing
- unsigned long flags;
- save_flags(flags); cli();
- ... critical section ...
- restore_flags(flags);
-so the code does _not_ need to worry about UP vs SMP issues: the spinlocks
-work correctly under both (and spinlocks are actually more efficient on
-architectures that allow doing the "save_flags + cli" in one operation).
+lock. This works well even under UP also, so the code does _not_ need to
+worry about UP vs SMP issues: the spinlocks work correctly under both.
NOTE! Implications of spin_locks for memory are further described in:
@@ -36,27 +26,7 @@ The above is usually pretty simple (you usually need and want only one
spinlock for most things - using more than one spinlock can make things a
lot more complex and even slower and is usually worth it only for
sequences that you _know_ need to be split up: avoid it at all cost if you
-aren't sure). HOWEVER, it _does_ mean that if you have some code that does
- cli();
- .. critical section ..
- sti();
-and another sequence that does
- spin_lock_irqsave(flags);
- .. critical section ..
- spin_unlock_irqrestore(flags);
-then they are NOT mutually exclusive, and the critical regions can happen
-at the same time on two different CPU's. That's fine per se, but the
-critical regions had better be critical for different things (ie they
-can't stomp on each other).
-The above is a problem mainly if you end up mixing code - for example the
-routines in ll_rw_block() tend to use cli/sti to protect the atomicity of
-their actions, and if a driver uses spinlocks instead then you should
-think about issues like the above.
+aren't sure).
This is really the only really hard part about spinlocks: once you start
using spinlocks they tend to expand to areas you might not have noticed
@@ -120,11 +90,10 @@ Lesson 3: spinlocks revisited.
The single spin-lock primitives above are by no means the only ones. They
are the most safe ones, and the ones that work under all circumstances,
-but partly _because_ they are safe they are also fairly slow. They are
-much faster than a generic global cli/sti pair, but slower than they'd
-need to be, because they do have to disable interrupts (which is just a
-single instruction on a x86, but it's an expensive one - and on other
-architectures it can be worse).
+but partly _because_ they are safe they are also fairly slow. They are slower
+than they'd need to be, because they do have to disable interrupts
+(which is just a single instruction on a x86, but it's an expensive one -
+and on other architectures it can be worse).
If you have a case where you have to protect a data structure across
several CPU's and you want to use spinlocks you can potentially use