From: "Paul E. McKenney" <[email protected]> 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: Paul E. McKenney <[email protected]> Cc: Linux-Arch <[email protected]> Cc: Ingo Molnar <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Oleg Nesterov <[email protected]> Cc: Linus Torvalds <[email protected]> --- include/linux/spinlock.h | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h index 75f34949d9ab..3f2867ff0ced 100644 --- a/include/linux/spinlock.h +++ b/include/linux/spinlock.h @@ -130,6 +130,16 @@ do { \ #define smp_mb__before_spinlock() smp_wmb() #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 * @lock: the spinlock in question. -- 1.8.1.5 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
