Re: [PATCH 5/5] futex: fix wakeup race by setting TASK_INTERRUPTIBLE before queue_me

Darren Hart
2009-09-22T17:22:20Z

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