>From 0ce84d118e2ee7ebc98ad4a8cfd23f04ad45115c Mon Sep 17 00:00:00 2001 From: Manfred Spraul Date: Sat, 10 Oct 2015 08:37:22 +0200 Subject: [PATCH] ipc/sem.c: Alternative for fixing Concurrency bug 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@informatik.uni-bremen.de Signed-off-by: Manfred Spraul --- 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