[PATCH] ipc/sem.c: Alternative for fixing Concurrency bug

[Manfred Spraul]

Two ideas for fixing the bug found by Felix:
- Revert my initial patch.
	Problem: Significant slowdown for application that use large sem
	arrays and complex operations: Every semop() does a loop
	with spin_lock() on all semaphores.

- Add another sem_wait_array() that catches operations that are in
  the middle of sem_lock().

What do you think? Is it worth to optimize for complex ops?

Reported-by: felixh@xxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
---
 ipc/sem.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/ipc/sem.c b/ipc/sem.c
index b471e5a..9a55cfb 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -1936,9 +1936,16 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
 			list_add_tail(&queue.list, &curr->pending_const);
 		}
 	} else {
-		if (!sma->complex_count)
+		if (!sma->complex_count) {
 			merge_queues(sma);
 
+			/*
+			 * squeeze out any simple operations that are in the middle
+			 * of sem_lock()
+			 */
+			sem_wait_array(sma);
+		}
+
 		if (alter)
 			list_add_tail(&queue.list, &sma->pending_alter);
 		else
-- 
2.4.3


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