From 0729e196147692d84d4c099fcff056eba2ed61d8 Mon Sep 17 00:00:00 2001 From: Darren Hart Date: Mon, 21 Sep 2009 22:30:38 -0700 Subject: [PATCH] futex: Fix wakeup race by setting TASK_INTERRUPTIBLE before queue_me() 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. Signed-off-by: Darren Hart Acked-by: Peter Zijlstra Cc: Steven Rostedt Cc: Eric Dumazet Cc: Dinakar Guniguntala Cc: John Stultz LKML-Reference: <20090922053038.8717.97838.stgit@Aeon> Signed-off-by: Ingo Molnar --- kernel/futex.c | 15 +++------------ 1 file changed, 3 insertions(+), 12 deletions(-) diff --git a/kernel/futex.c b/kernel/futex.c index f92afbe3d3a1..463af2efa512 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1656,17 +1656,8 @@ static int fixup_owner(u32 __user *uaddr, int fshared, struct futex_q *q, 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); + 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))) { /*