ipc/sem.c: always use only one queue for alter operations

There are two places that can contain alter operations:
- the global queue: sma->pending_alter
- the per-semaphore queues: sma->sem_base[].pending_alter.

Since one of the queues must be processed first, this causes an odd
priorization of the wakeups:
Right now, complex operations have priority over simple ops.

The patch restores the behavior of linux <=3.0.9: The longest
waiting operation has the highest priority.

This is done by using only one queue:
- if there are complex ops, then sma->pending_alter is used.
- otherwise, the per-semaphore queues are used.

As a side effect, do_smart_update_queue() becomes much simpler:
No more goto logic.

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Davidlohr Bueso <davidlohr.bueso@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/ipc/sem.c b/ipc/sem.c
index 4d7f88cefada0c9c3edbe1265133adbfc1ef34dd..6291257ee0498e9976e1d0bf5bcea250ee95f93d 100644 (file)
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -192,6 +192,53 @@ void __init sem_init (void)
                                IPC_SEM_IDS, sysvipc_sem_proc_show);
 }
 
+/**
+ * unmerge_queues - unmerge queues, if possible.
+ * @sma: semaphore array
+ *
+ * The function unmerges the wait queues if complex_count is 0.
+ * It must be called prior to dropping the global semaphore array lock.
+ */
+static void unmerge_queues(struct sem_array *sma)
+{
+       struct sem_queue *q, *tq;
+
+       /* complex operations still around? */
+       if (sma->complex_count)
+               return;
+       /*
+        * We will switch back to simple mode.
+        * Move all pending operation back into the per-semaphore
+        * queues.
+        */
+       list_for_each_entry_safe(q, tq, &sma->pending_alter, list) {
+               struct sem *curr;
+               curr = &sma->sem_base[q->sops[0].sem_num];
+
+               list_add_tail(&q->list, &curr->pending_alter);
+       }
+       INIT_LIST_HEAD(&sma->pending_alter);
+}
+
+/**
+ * merge_queues - Merge single semop queues into global queue
+ * @sma: semaphore array
+ *
+ * This function merges all per-semaphore queues into the global queue.
+ * It is necessary to achieve FIFO ordering for the pending single-sop
+ * operations when a multi-semop operation must sleep.
+ * Only the alter operations must be moved, the const operations can stay.
+ */
+static void merge_queues(struct sem_array *sma)
+{
+       int i;
+       for (i = 0; i < sma->sem_nsems; i++) {
+               struct sem *sem = sma->sem_base + i;
+
+               list_splice_init(&sem->pending_alter, &sma->pending_alter);
+       }
+}
+
 /*
  * If the request contains only one semaphore operation, and there are
  * no complex transactions pending, lock only the semaphore involved.
@@ -262,6 +309,7 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
 static inline void sem_unlock(struct sem_array *sma, int locknum)
 {
        if (locknum == -1) {
+               unmerge_queues(sma);
                ipc_unlock_object(&sma->sem_perm);
        } else {
                struct sem *sem = sma->sem_base + locknum;
@@ -831,49 +879,38 @@ static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsop
                        int otime, struct list_head *pt)
 {
        int i;
-       int progress;
 
        otime |= do_smart_wakeup_zero(sma, sops, nsops, pt);
 
-       progress = 1;
-retry_global:
-       if (sma->complex_count) {
-               if (update_queue(sma, -1, pt)) {
-                       progress = 1;
-                       otime = 1;
-                       sops = NULL;
-               }
-       }
-       if (!progress)
-               goto done;
-
-       if (!sops) {
-               /* No semops; something special is going on. */
-               for (i = 0; i < sma->sem_nsems; i++) {
-                       if (update_queue(sma, i, pt)) {
-                               otime = 1;
-                               progress = 1;
+       if (!list_empty(&sma->pending_alter)) {
+               /* semaphore array uses the global queue - just process it. */
+               otime |= update_queue(sma, -1, pt);
+       } else {
+               if (!sops) {
+                       /*
+                        * No sops, thus the modified semaphores are not
+                        * known. Check all.
+                        */
+                       for (i = 0; i < sma->sem_nsems; i++)
+                               otime |= update_queue(sma, i, pt);
+               } else {
+                       /*
+                        * Check the semaphores that were increased:
+                        * - No complex ops, thus all sleeping ops are
+                        *   decrease.
+                        * - if we decreased the value, then any sleeping
+                        *   semaphore ops wont be able to run: If the
+                        *   previous value was too small, then the new
+                        *   value will be too small, too.
+                        */
+                       for (i = 0; i < nsops; i++) {
+                               if (sops[i].sem_op > 0) {
+                                       otime |= update_queue(sma,
+                                                       sops[i].sem_num, pt);
+                               }
                        }
                }
-               goto done_checkretry;
-       }
-
-       /* Check the semaphores that were modified. */
-       for (i = 0; i < nsops; i++) {
-               if (sops[i].sem_op > 0 ||
-                       (sops[i].sem_op < 0 &&
-                               sma->sem_base[sops[i].sem_num].semval == 0))
-                       if (update_queue(sma, sops[i].sem_num, pt)) {
-                               otime = 1;
-                               progress = 1;
-                       }
-       }
-done_checkretry:
-       if (progress) {
-               progress = 0;
-               goto retry_global;
        }
-done:
        if (otime)
                sma->sem_otime = get_seconds();
 }
@@ -1747,11 +1784,22 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
                struct sem *curr;
                curr = &sma->sem_base[sops->sem_num];
 
-               if (alter)
-                       list_add_tail(&queue.list, &curr->pending_alter);
-               else
+               if (alter) {
+                       if (sma->complex_count) {
+                               list_add_tail(&queue.list,
+                                               &sma->pending_alter);
+                       } else {
+
+                               list_add_tail(&queue.list,
+                                               &curr->pending_alter);
+                       }
+               } else {
                        list_add_tail(&queue.list, &curr->pending_const);
+               }
        } else {
+               if (!sma->complex_count)
+                       merge_queues(sma);
+
                if (alter)
                        list_add_tail(&queue.list, &sma->pending_alter);
                else