Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752707AbZIVRWU (ORCPT ); Tue, 22 Sep 2009 13:22:20 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751277AbZIVRWU (ORCPT ); Tue, 22 Sep 2009 13:22:20 -0400 Received: from e37.co.us.ibm.com ([32.97.110.158]:48941 "EHLO e37.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750984AbZIVRWT (ORCPT ); Tue, 22 Sep 2009 13:22:19 -0400 Message-ID: <4AB907AD.1020605@us.ibm.com> Date: Tue, 22 Sep 2009 10:21:49 -0700 From: Darren Hart User-Agent: Thunderbird 2.0.0.23 (X11/20090817) MIME-Version: 1.0 To: Eric Dumazet CC: linux-kernel@vger.kernel.org, Thomas Gleixner , Peter Zijlstra , Steven Rostedt , Ingo Molnar , Dinakar Guniguntala , John Stultz Subject: Re: [PATCH 5/5] futex: fix wakeup race by setting TASK_INTERRUPTIBLE before queue_me References: <20090922052452.8717.39673.stgit@Aeon> <20090922053038.8717.97838.stgit@Aeon> <4AB88D09.3080907@gmail.com> In-Reply-To: <4AB88D09.3080907@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4399 Lines: 127 Eric Dumazet wrote: > Darren Hart a écrit : >> PI futexes do not use the same plist_node_empty() test for wakeup. It was >> possible for the waiter (in futex_wait_requeue_pi()) to set TASK_INTERRUPTIBLE >> after the waker assigned the rtmutex to the waiter. The waiter would then note >> the plist was not empty and call schedule(). The task would not be found by any >> subsequeuent futex wakeups, resulting in a userspace hang. By moving the >> setting of TASK_INTERRUPTIBLE to before the call to queue_me(), the race with >> the waker is eliminated. Since we no longer call get_user() from within >> queue_me(), there is no need to delay the setting of TASK_INTERRUPTIBLE until >> after the call to queue_me(). >> >> The FUTEX_LOCK_PI operation is not affected as futex_lock_pi() relies entirely >> on the rtmutex code to handle schedule() and wakeup. The requeue PI code is >> affected because the waiter starts as a non-PI waiter and is woken on a PI >> futex. >> >> Remove the crusty old comment about holding spinlocks() across get_user() as we >> no longer do that. Correct the locking statement with a description of why the >> test is performed. > > I am very confused by this ChangeLog... > >> Signed-off-by: Darren Hart >> Cc: Thomas Gleixner >> Cc: Peter Zijlstra >> Cc: Steven Rostedt >> Cc: Ingo Molnar >> CC: Eric Dumazet >> CC: Dinakar Guniguntala >> CC: John Stultz >> --- >> >> kernel/futex.c | 15 +++------------ >> 1 files changed, 3 insertions(+), 12 deletions(-) >> >> diff --git a/kernel/futex.c b/kernel/futex.c >> index f92afbe..463af2e 100644 >> --- a/kernel/futex.c >> +++ b/kernel/futex.c >> @@ -1656,17 +1656,8 @@ out: >> static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q, >> struct hrtimer_sleeper *timeout) >> { >> - queue_me(q, hb); >> - >> - /* >> - * There might have been scheduling since the queue_me(), as we >> - * cannot hold a spinlock across the get_user() in case it >> - * faults, and we cannot just set TASK_INTERRUPTIBLE state when >> - * queueing ourselves into the futex hash. This code thus has to >> - * rely on the futex_wake() code removing us from hash when it >> - * wakes us up. >> - */ >> set_current_state(TASK_INTERRUPTIBLE); > > Hmm, you missed the smp_mb() properties here... > > Before : > queue_me() > set_mb(current->state, TASK_INTERRUPTIBLE); > if (timeout) {...} > if (likely(!plist_node_empty(&q->list))) { > ... > } > > After : > set_mb(current->state, TASK_INTERRUPTIBLE); > queue_me(); > if (timeout) {...} > // no barrier... why ar we still testing q->list > // since it has no synchro between queue_me() and test ? As Ingo said, the barrier is covered by the spin_unlock() in queue_me() according to memory-barriers.txt: (2) UNLOCK operation implication: Memory operations issued before the UNLOCK will be completed before the UNLOCK operation has completed. > if (likely(!plist_node_empty(&q->list))) { Note that this test is really just an optimization to avoid calling schedule() if the waker has already removed the futex_q from the list. If it is about to wake us, but hasn't removed us from the list, it will have set TASK_RUNNING and schedule() will do the right thing, with a little more overhead than is truly necessary. Thanks, Darren Hart > ... > } > > > >> + queue_me(q, hb); >> >> /* Arm the timer */ >> if (timeout) { >> @@ -1676,8 +1667,8 @@ static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q, >> } >> >> /* >> - * !plist_node_empty() is safe here without any lock. >> - * q.lock_ptr != 0 is not safe, because of ordering against wakeup. >> + * If we have been removed from the hash list, then another task >> + * has tried to wake us, and we can skip the call to schedule(). >> */ >> if (likely(!plist_node_empty(&q->list))) { >> /* >> > -- Darren Hart IBM Linux Technology Center Real-Time Linux Team -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/