Messages in this thread Patch in this message | | | From | Manfred Spraul <> | Subject | [PATCH] ipc/sem.c: [RFC] memory barrier in sem_lock() | Date | Tue, 12 Aug 2014 21:43:06 +0200 |
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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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