Messages in this thread | | | Date | Tue, 22 Sep 2009 21:26:49 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 5/5] futex: fix wakeup race by setting TASK_INTERRUPTIBLE before queue_me |
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* Darren Hart <dvhltc@us.ibm.com> wrote:
> Eric Dumazet 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 ? > > 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.
btw., it might make sense to add a comment about this - it's not trivial at all and the barrier rules here are tricky ...
Ingo
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