[PATCH 2/2] ipc/sem.c: optimize sem_lock().

[Manfred Spraul]

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.

What do you think?
The patch survived some testing.

Its not a bugfix - thus I don't know if it should go into linux-next first.

Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Cc: Mike Galbraith <bitbucket@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Davidlohr Bueso <davidlohr.bueso@xxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
 ipc/sem.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/ipc/sem.c b/ipc/sem.c
index 4836ea7..5274ed1 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -248,12 +248,20 @@ static void merge_queues(struct sem_array *sma)
  * Caller must own sem_perm.lock.
  * New simple ops can 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);
-- 
1.8.3.1

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