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

[Darren Hart <dvhltc@us.ibm.com>]

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