Commit 6d07b68c authored Sep 30, 2013 by Manfred Spraul Committed by Linus Torvalds Sep 30, 2013

ipc/sem.c: optimize sem_lock()



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>

parent 5e9d5275

ipc/sem.c

+8 −0

Original line number	Diff line number	Diff line
		@@ -257,12 +257,20 @@ static void sem_rcu_free(struct rcu_head *head)
		* 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);

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