Commit 05801817 authored by Muthu Kumar's avatar Muthu Kumar Committed by Linus Torvalds

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: default avatarMuthukumar R <muthur@gmail.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent e3bbfa78
......@@ -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
......
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