[PATCH] ipc/sem.c: [RFC] memory barrier in sem_lock()

[Manfred Spraul]

sem_lock right now contains an smp_mb().
I think smp_rmb() would be sufficient - and performance of semop() with rmb()
is up to 10% faster. It would be a pairing of rmb() with spin_unlock().

The race we must protect against is:

sem->lock is free
sma->complex_count = 0
sma->sem_perm.lock held by thread B

thread A:

A: spin_lock(&sem->lock)

			B: sma->complex_count++; (now 1)
			B: spin_unlock(&sma->sem_perm.lock);

A: spin_is_locked(&sma->sem_perm.lock);
A: XXXXX which memory barrier is necessary?
A: if (sma->complex_count == 0)

Thread A must read the increased complex_count value, i.e. the read must
not be reordered with the read of sem_perm.lock done by spin_is_locked().

But that's it, there are no writes that must be ordered.

---
 ipc/sem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/ipc/sem.c b/ipc/sem.c
index 454f6c6..a5c8a77 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -327,7 +327,7 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
 		/* Then check that the global lock is free */
 		if (!spin_is_locked(&sma->sem_perm.lock)) {
 			/* spin_is_locked() is not a memory barrier */
-			smp_mb();
+			smp_rmb();
 
 			/* Now repeat the test of complex_count:
 			 * It can't change anymore until we drop sem->lock.
-- 
1.9.3

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