Messages in this thread Patch in this message | | | Subject | Re: PROBLEM: Concurrency issue in sem_lock | From | Manfred Spraul <> | Date | Sat, 10 Oct 2015 08:50:22 +0200 |
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Hi,
On 10/09/2015 10:24 AM, Felix Hübner wrote: > Hi all, > > I have just reported a concurrency issue in the implementation of > sem_lock, see https://bugzilla.kernel.org/show_bug.cgi?id=105651 > > [...] > # P0 does spin_lock(&sem->lock); in line 336. > > spin_lock(&sem->lock); [...] > # P2 performs rest of semtimedop, increments complex_count and ends up > in line 1961 and starts to sleep. > > return -1; > } That is the problem: semtimedop() increments complex_count - thus sem_wait_array() returns without a spin_unlock_wait() loop - but P0 already owns spin_lock(&sem->lock).
How do we want to fix it? - revert my patch (simplify code, but slower for one corner case) - add the missing sem_wait_array (more complex, but also better for complex semops).
what do you think?
(patch untested)
-- Manfred From 0ce84d118e2ee7ebc98ad4a8cfd23f04ad45115c Mon Sep 17 00:00:00 2001 From: Manfred Spraul <manfred@colorfullife.com> 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 <manfred@colorfullife.com> --- 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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