From 2e094abfd1f29a08a60523b42d4508281b8dee0e Mon Sep 17 00:00:00 2001 From: Manfred Spraul Date: Fri, 12 Dec 2014 16:58:11 -0800 Subject: [PATCH] ipc/sem.c: change memory barrier in sem_lock() to smp_rmb() When I fixed bugs in the sem_lock() logic, I was more conservative than necessary. Therefore it is safe to replace the smp_mb() with smp_rmb(). And: With smp_rmb(), semop() syscalls are up to 10% faster. 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 memory barrier 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(). Since it's about ordering of reads, smp_rmb() is sufficient. [akpm@linux-foundation.org: update sem_lock() comment, from Davidlohr] Signed-off-by: Manfred Spraul Reviewed-by: Davidlohr Bueso Acked-by: Rafael Aquini Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- ipc/sem.c | 13 ++++++++++--- 1 file changed, 10 insertions(+), 3 deletions(-) diff --git a/ipc/sem.c b/ipc/sem.c index 53c3310f41c6..6115146563f9 100644 --- a/ipc/sem.c +++ b/ipc/sem.c @@ -326,10 +326,17 @@ 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(); + /* + * The ipc object lock check must be visible on all + * cores before rechecking the complex count. Otherwise + * we can race with another thread that does: + * complex_count++; + * spin_unlock(sem_perm.lock); + */ + smp_rmb(); - /* Now repeat the test of complex_count: + /* + * Now repeat the test of complex_count: * It can't change anymore until we drop sem->lock. * Thus: if is now 0, then it will stay 0. */ -- 2.30.2