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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/