Messages in this thread | | | Date | Tue, 22 Sep 2009 11:10:45 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 5/5] futex: fix wakeup race by setting TASK_INTERRUPTIBLE before queue_me |
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* 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
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