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ipc/sem.c: optimize sem_lock()

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Operations that need access to the whole array must guarantee that there
are no simple operations ongoing.  Right now this is achieved by
spin_unlock_wait(sem->lock) on all semaphores.

If complex_count is nonzero, then this spin_unlock_wait() is not
necessary, because it was already performed in the past by the thread
that increased complex_count and even though sem_perm.lock was dropped
inbetween, no simple operation could have started, because simple
operations cannot start when complex_count is non-zero.

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Mike Galbraith <bitbucket@online.de>
Cc: Rik van Riel <riel@redhat.com>
Reviewed-by: Davidlohr Bueso <davidlohr@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 2013-09-30 13:45:06 -07:00 committed by Linus Torvalds
parent 5e9d527591
commit 6d07b68ce1
1 changed files with 8 additions and 0 deletions
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ipc/sem.c
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@ -257,12 +257,20 @@ static void sem_rcu_free(struct rcu_head *head)
* Caller must own sem_perm.lock. * Caller must own sem_perm.lock.
* New simple ops cannot start, because simple ops first check * New simple ops cannot start, because simple ops first check
* that sem_perm.lock is free. * 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) static void sem_wait_array(struct sem_array *sma)
{ {
int i; int i;
struct sem *sem; 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++) { for (i = 0; i < sma->sem_nsems; i++) {
sem = sma->sem_base + i; sem = sma->sem_base + i;
spin_unlock_wait(&sem->lock); spin_unlock_wait(&sem->lock);