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Subject[ 44/69] ipc/sem.c: always use only one queue for alter operations
Date
3.10-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Manfred Spraul <manfred@colorfullife.com>

commit f269f40ad5aeee229ed70044926f44318abe41ef upstream.

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: 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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
ipc/sem.c | 128 ++++++++++++++++++++++++++++++++++++++++++--------------------
1 file changed, 88 insertions(+), 40 deletions(-)

--- 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_ar
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_a
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 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



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