X-Git-Url: http://demsky.eecs.uci.edu/git/?a=blobdiff_plain;f=ipc%2Fsem.c;h=6115146563f94e5bfab0973be730b3388fb3fc59;hb=b3890e4704594fa23abe1395d1fafc97d3214be8;hp=53c3310f41c6867fd4b5f3f0493a150184b8c617;hpb=865566ed33f08637c3486b4d301e96a9064ad2eb;p=firefly-linux-kernel-4.4.55.git

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.
 			 */