linux-kernel - [ 47/50] ipc/sem.c: optimize sem

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Message-Id: <20131016174404.122710496@linuxfoundation.org>
Date:	Wed, 16 Oct 2013 10:45:33 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Manfred Spraul <manfred@...orfullife.com>,
	Mike Galbraith <bitbucket@...ine.de>,
	Rik van Riel <riel@...hat.com>,
	Davidlohr Bueso <davidlohr@...com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mike Galbraith <efault@....de>
Subject: [ 47/50] ipc/sem.c: optimize sem_lock()

3.11-stable review patch.  If anyone has any objections, please let me know.

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

From: Manfred Spraul <manfred@...orfullife.com>

commit 6d07b68ce16ae9535955ba2059dedba5309c3ca1 upstream.

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@...orfullife.com>
Cc: Mike Galbraith <bitbucket@...ine.de>
Cc: Rik van Riel <riel@...hat.com>
Reviewed-by: Davidlohr Bueso <davidlohr@...com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mike Galbraith <efault@....de>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 ipc/sem.c |    8 ++++++++
 1 file changed, 8 insertions(+)

--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -248,12 +248,20 @@ static void merge_queues(struct sem_arra
  * 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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