ipc/sem.c: change memory barrier in sem_lock() to smp_rmb()

[karo-tx-linux.git] / ipc / sem.c

diff --git a/ipc/sem.c b/ipc/sem.c
index 53c3310f41c6867fd4b5f3f0493a150184b8c617..6115146563f94e5bfab0973be730b3388fb3fc59 100644 (file)
--- 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.
                         */