Messages in this thread | | | Date | Tue, 22 Sep 2009 10:21:49 -0700 | From | Darren Hart <> | Subject | Re: [PATCH 5/5] futex: fix wakeup race by setting TASK_INTERRUPTIBLE before queue_me |
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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 <dvhltc@us.ibm.com> >> Cc: Thomas Gleixner <tglx@linutronix.de> >> Cc: Peter Zijlstra <peterz@infradead.org> >> Cc: Steven Rostedt <rostedt@goodmis.org> >> Cc: Ingo Molnar <mingo@elte.hu> >> CC: Eric Dumazet <eric.dumazet@gmail.com> >> CC: Dinakar Guniguntala <dino@in.ibm.com> >> CC: John Stultz <johnstul@us.ibm.com> >> --- >> >> 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/
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