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

Eric Dumazet
2009-09-22T08:38:59Z

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 ?
if (likely(!plist_node_empty(&q->list))) {
...
}

> + 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))) {
> /*
>