Messages in this thread Patch in this message | | | Date | Tue, 22 Sep 2009 10:38:33 +0200 | From | Eric Dumazet <> | Subject | Re: [PATCH 5/5] futex: fix wakeup race by setting TASK_INTERRUPTIBLE before queue_me |
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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))) { > /* >
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