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

[Ingo Molnar <mingo@elte.hu>]

* Eric Dumazet <eric.dumazet@gmail.com> wrote:

> Darren Hart a ??crit :
> > 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 ?
>      if (likely(!plist_node_empty(&q->list))) {
> 	...
>      }

queue_me() itself does a spin_unlock(), so at least for the bits 
protected by hb->lock it should be half-serializing.

	Ingo