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futex: Fix (possible) missed wakeup

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We must not rely on wake_q_add() to delay the wakeup; in particular
commit:

  1d0dcb3ad9 ("futex: Implement lockless wakeups")

moved wake_q_add() before smp_store_release(&q->lock_ptr, NULL), which
could result in futex_wait() waking before observing ->lock_ptr ==
NULL and going back to sleep again.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 1d0dcb3ad9 ("futex: Implement lockless wakeups")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2018-11-29 14:44:49 +01:00 committed by Ingo Molnar
parent 4c4e373156
commit b061c38bef
1 changed files with 8 additions and 5 deletions
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13
kernel/futex.c
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@ -1452,11 +1452,7 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q)
if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n"))
return;
/*
* Queue the task for later wakeup for after we've released
* the hb->lock. wake_q_add() grabs reference to p.
*/
wake_q_add(wake_q, p);
get_task_struct(p);
__unqueue_futex(q);
/*
* The waiting task can free the futex_q as soon as q->lock_ptr = NULL
@ -1466,6 +1462,13 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q)
* plist_del in __unqueue_futex().
*/
smp_store_release(&q->lock_ptr, NULL);
/*
* Queue the task for later wakeup for after we've released
* the hb->lock. wake_q_add() grabs reference to p.
*/
wake_q_add(wake_q, p);
put_task_struct(p);
}
/*