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

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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>
This commit is contained in:
Manfred Spraul 2013-07-08 16:01:24 -07:00 committed by Linus Torvalds
parent 1a82e9e1d0
commit f269f40ad5
1 changed files with 88 additions and 40 deletions
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128
ipc/sem.c
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@ -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