1 files changed, 123 insertions, 57 deletions

diff --git a/ipc/sem.c b/ipc/sem.c
index 19c8b980d1fe..8c4f59b0204a 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -253,70 +253,112 @@ static void sem_rcu_free(struct rcu_head *head)
 }
 
 /*
+ * Wait until all currently ongoing simple ops have completed.
+ * Caller must own sem_perm.lock.
+ * New simple ops cannot start, because simple ops first check
+ * that sem_perm.lock is free.
+ * that a) sem_perm.lock is free and b) complex_count is 0.
+ */
+static void sem_wait_array(struct sem_array *sma)
+{
+        int i;
+        struct sem *sem;
+
+        if (sma->complex_count)  {
+                /* The thread that increased sma->complex_count waited on
+                 * all sem->lock locks. Thus we don't need to wait again.
+                 */
+                return;
+        }
+
+        for (i = 0; i < sma->sem_nsems; i++) {
+                sem = sma->sem_base + i;
+                spin_unlock_wait(&sem->lock);
+        }
+}
+
+/*
  * If the request contains only one semaphore operation, and there are
  * no complex transactions pending, lock only the semaphore involved.
  * Otherwise, lock the entire semaphore array, since we either have
  * multiple semaphores in our own semops, or we need to look at
  * semaphores from other pending complex operations.
- *
- * Carefully guard against sma->complex_count changing between zero
- * and non-zero while we are spinning for the lock. The value of
- * sma->complex_count cannot change while we are holding the lock,
- * so sem_unlock should be fine.
- *
- * The global lock path checks that all the local locks have been released,
- * checking each local lock once. This means that the local lock paths
- * cannot start their critical sections while the global lock is held.
  */
 static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
                               int nsops)
 {
-        int locknum;
- again:
-        if (nsops == 1 && !sma->complex_count) {
-                struct sem *sem = sma->sem_base + sops->sem_num;
+        struct sem *sem;
 
-                /* Lock just the semaphore we are interested in. */
-                spin_lock(&sem->lock);
+        if (nsops != 1) {
+                /* Complex operation - acquire a full lock */
+                ipc_lock_object(&sma->sem_perm);
 
-                /*
-                 * If sma->complex_count was set while we were spinning,
-                 * we may need to look at things we did not lock here.
+                /* And wait until all simple ops that are processed
+                 * right now have dropped their locks.
                  */
-                if (unlikely(sma->complex_count)) {
-                        spin_unlock(&sem->lock);
-                        goto lock_array;
-                }
+                sem_wait_array(sma);
+                return -1;
+        }
 
+        /*
+         * Only one semaphore affected - try to optimize locking.
+         * The rules are:
+         * - optimized locking is possible if no complex operation
+         *   is either enqueued or processed right now.
+         * - The test for enqueued complex ops is simple:
+         *      sma->complex_count != 0
+         * - Testing for complex ops that are processed right now is
+         *   a bit more difficult. Complex ops acquire the full lock
+         *   and first wait that the running simple ops have completed.
+         *   (see above)
+         *   Thus: If we own a simple lock and the global lock is free
+         *      and complex_count is now 0, then it will stay 0 and
+         *      thus just locking sem->lock is sufficient.
+         */
+        sem = sma->sem_base + sops->sem_num;
+
+        if (sma->complex_count == 0) {
                 /*
-                 * Another process is holding the global lock on the
-                 * sem_array; we cannot enter our critical section,
-                 * but have to wait for the global lock to be released.
+                 * It appears that no complex operation is around.
+                 * Acquire the per-semaphore lock.
                  */
-                if (unlikely(spin_is_locked(&sma->sem_perm.lock))) {
-                        spin_unlock(&sem->lock);
-                        spin_unlock_wait(&sma->sem_perm.lock);
-                        goto again;
+                spin_lock(&sem->lock);
+
+                /* 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();
+
+                        /* 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.
+                         */
+                        if (sma->complex_count == 0) {
+                                /* fast path successful! */
+                                return sops->sem_num;
+                        }
                 }
+                spin_unlock(&sem->lock);
+        }
 
-                locknum = sops->sem_num;
+        /* slow path: acquire the full lock */
+        ipc_lock_object(&sma->sem_perm);
+
+        if (sma->complex_count == 0) {
+                /* False alarm:
+                 * There is no complex operation, thus we can switch
+                 * back to the fast path.
+                 */
+                spin_lock(&sem->lock);
+                ipc_unlock_object(&sma->sem_perm);
+                return sops->sem_num;
         } else {
-                int i;
-                /*
-                 * Lock the semaphore array, and wait for all of the
-                 * individual semaphore locks to go away.  The code
-                 * above ensures no new single-lock holders will enter
-                 * their critical section while the array lock is held.
+                /* Not a false alarm, thus complete the sequence for a
+                 * full lock.
                  */
- lock_array:
-                ipc_lock_object(&sma->sem_perm);
-                for (i = 0; i < sma->sem_nsems; i++) {
-                        struct sem *sem = sma->sem_base + i;
-                        spin_unlock_wait(&sem->lock);
-                }
-                locknum = -1;
+                sem_wait_array(sma);
+                return -1;
         }
-        return locknum;
 }
 
 static inline void sem_unlock(struct sem_array *sma, int locknum)
@@ -876,6 +918,24 @@ again:
 }
 
 /**
+ * set_semotime(sma, sops) - set sem_otime
+ * @sma: semaphore array
+ * @sops: operations that modified the array, may be NULL
+ *
+ * sem_otime is replicated to avoid cache line trashing.
+ * This function sets one instance to the current time.
+ */
+static void set_semotime(struct sem_array *sma, struct sembuf *sops)
+{
+        if (sops == NULL) {
+                sma->sem_base[0].sem_otime = get_seconds();
+        } else {
+                sma->sem_base[sops[0].sem_num].sem_otime =
+                                                        get_seconds();
+        }
+}
+
+/**
  * do_smart_update(sma, sops, nsops, otime, pt) - optimized update_queue
  * @sma: semaphore array
  * @sops: operations that were performed
@@ -925,17 +985,10 @@ static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsop
                         }
                 }
         }
-        if (otime) {
-                if (sops == NULL) {
-                        sma->sem_base[0].sem_otime = get_seconds();
-                } else {
-                        sma->sem_base[sops[0].sem_num].sem_otime =
-                                                                get_seconds();
-                }
-        }
+        if (otime)
+                set_semotime(sma, sops);
 }
 
-
 /* The following counts are associated to each semaphore:
  *   semncnt        number of tasks waiting on semval being nonzero
  *   semzcnt        number of tasks waiting on semval being zero
@@ -1797,12 +1850,17 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
 
         error = perform_atomic_semop(sma, sops, nsops, un,
                                         task_tgid_vnr(current));
-        if (error <= 0) {
-                if (alter && error == 0)
+        if (error == 0) {
+                /* If the operation was successful, then do
+                 * the required updates.
+                 */
+                if (alter)
                         do_smart_update(sma, sops, nsops, 1, &tasks);
-
-                goto out_unlock_free;
+                else
+                        set_semotime(sma, sops);
         }
+        if (error <= 0)
+                goto out_unlock_free;
 
         /* We need to sleep on this operation, so we put the current
          * task into the pending queue and go to sleep.
@@ -2061,6 +2119,14 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it)
         struct sem_array *sma = it;
         time_t sem_otime;
 
+        /*
+         * The proc interface isn't aware of sem_lock(), it calls
+         * ipc_lock_object() directly (in sysvipc_find_ipc).
+         * In order to stay compatible with sem_lock(), we must wait until
+         * all simple semop() calls have left their critical regions.
+         */
+        sem_wait_array(sma);
+
         sem_otime = get_semotime(sma);
 
         return seq_printf(s,