Commit 01352fb8 authored by Paul E. McKenney's avatar Paul E. McKenney Committed by Ingo Molnar

locking: Add an smp_mb__after_unlock_lock() for UNLOCK+BLOCK barrier

The Linux kernel has traditionally required that an UNLOCK+LOCK
pair act as a full memory barrier when either (1) that
UNLOCK+LOCK pair was executed by the same CPU or task, or (2)
the same lock variable was used for the UNLOCK and LOCK.  It now
seems likely that very few places in the kernel rely on this
full-memory-barrier semantic, and with the advent of queued
locks, providing this semantic either requires complex
reasoning, or for some architectures, added overhead.

This commit therefore adds a smp_mb__after_unlock_lock(), which
may be placed after a LOCK primitive to restore the
full-memory-barrier semantic. All definitions are currently
no-ops, but will be upgraded for some architectures when queued
locks arrive.
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
Cc: <linux-arch@vger.kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/1386799151-2219-5-git-send-email-paulmck@linux.vnet.ibm.comSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 692118da
...@@ -130,6 +130,16 @@ do { \ ...@@ -130,6 +130,16 @@ do { \
#define smp_mb__before_spinlock() smp_wmb() #define smp_mb__before_spinlock() smp_wmb()
#endif #endif
/*
* Place this after a lock-acquisition primitive to guarantee that
* an UNLOCK+LOCK pair act as a full barrier. This guarantee applies
* if the UNLOCK and LOCK are executed by the same CPU or if the
* UNLOCK and LOCK operate on the same lock variable.
*/
#ifndef smp_mb__after_unlock_lock
#define smp_mb__after_unlock_lock() do { } while (0)
#endif
/** /**
* raw_spin_unlock_wait - wait until the spinlock gets unlocked * raw_spin_unlock_wait - wait until the spinlock gets unlocked
* @lock: the spinlock in question. * @lock: the spinlock in question.
......
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment