ipc/sem.c: use READ_ONCE()/WRITE_ONCE() for use_global_lock

[linux-2.6-block.git] / ipc / sem.c

diff --git a/ipc/sem.c b/ipc/sem.c
index 3a58188733d80e7dbccc4a6e3e44d6708e1f9dcd..971e75d28364e5a3c2a64f9bfc9bbf5bcaef91c9 100644 (file)
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -217,6 +217,8 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it);
  * this smp_load_acquire(), this is guaranteed because the smp_load_acquire()
  * is inside a spin_lock() and after a write from 0 to non-zero a
  * spin_lock()+spin_unlock() is done.
+ * To prevent the compiler/cpu temporarily writing 0 to use_global_lock,
+ * READ_ONCE()/WRITE_ONCE() is used.
  *
  * 2) queue.status: (SEM_BARRIER_2)
  * Initialization is done while holding sem_lock(), so no further barrier is
@@ -342,10 +344,10 @@ static void complexmode_enter(struct sem_array *sma)
                 * Nothing to do, just reset the
                 * counter until we return to simple mode.
                 */
-               sma->use_global_lock = USE_GLOBAL_LOCK_HYSTERESIS;
+               WRITE_ONCE(sma->use_global_lock, USE_GLOBAL_LOCK_HYSTERESIS);
                return;
        }
-       sma->use_global_lock = USE_GLOBAL_LOCK_HYSTERESIS;
+       WRITE_ONCE(sma->use_global_lock, USE_GLOBAL_LOCK_HYSTERESIS);
 
        for (i = 0; i < sma->sem_nsems; i++) {
                sem = &sma->sems[i];
@@ -371,7 +373,8 @@ static void complexmode_tryleave(struct sem_array *sma)
                /* See SEM_BARRIER_1 for purpose/pairing */
                smp_store_release(&sma->use_global_lock, 0);
        } else {
-               sma->use_global_lock--;
+               WRITE_ONCE(sma->use_global_lock,
+                               sma->use_global_lock-1);
        }
 }
 
@@ -412,7 +415,7 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
         * Initial check for use_global_lock. Just an optimization,
         * no locking, no memory barrier.
         */
-       if (!sma->use_global_lock) {
+       if (!READ_ONCE(sma->use_global_lock)) {
                /*
                 * It appears that no complex operation is around.
                 * Acquire the per-semaphore lock.
@@ -2436,7 +2439,8 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it)
 
        /*
         * The proc interface isn't aware of sem_lock(), it calls
-        * ipc_lock_object() directly (in sysvipc_find_ipc).
+        * ipc_lock_object(), i.e. spin_lock(&sma->sem_perm.lock).
+        * (in sysvipc_find_ipc)
         * In order to stay compatible with sem_lock(), we must
         * enter / leave complex_mode.
         */