futex: Fix (possible) missed wakeup
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>
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@ -1452,11 +1452,7 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q)
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if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n"))
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if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n"))
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return;
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return;
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/*
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get_task_struct(p);
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* Queue the task for later wakeup for after we've released
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* the hb->lock. wake_q_add() grabs reference to p.
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*/
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wake_q_add(wake_q, p);
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__unqueue_futex(q);
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__unqueue_futex(q);
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/*
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/*
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* The waiting task can free the futex_q as soon as q->lock_ptr = NULL
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* The waiting task can free the futex_q as soon as q->lock_ptr = NULL
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@ -1466,6 +1462,13 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q)
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* plist_del in __unqueue_futex().
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* plist_del in __unqueue_futex().
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*/
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*/
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smp_store_release(&q->lock_ptr, NULL);
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smp_store_release(&q->lock_ptr, NULL);
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/*
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* Queue the task for later wakeup for after we've released
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* the hb->lock. wake_q_add() grabs reference to p.
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*/
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wake_q_add(wake_q, p);
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put_task_struct(p);
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}
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}
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/*
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/*
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