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    Subject[PATCH AUTOSEL 4.14 20/40] futex: Fix (possible) missed wakeup
    Date
    From: Peter Zijlstra <peterz@infradead.org>

    [ Upstream commit b061c38bef43406df8e73c5be06cbfacad5ee6ad ]

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

    1d0dcb3ad9d3 ("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: 1d0dcb3ad9d3 ("futex: Implement lockless wakeups")
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/futex.c | 13 ++++++++-----
    1 file changed, 8 insertions(+), 5 deletions(-)

    diff --git a/kernel/futex.c b/kernel/futex.c
    index 046cd780d057..176913206579 100644
    --- a/kernel/futex.c
    +++ b/kernel/futex.c
    @@ -1405,11 +1405,7 @@ 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
    @@ -1419,6 +1415,13 @@ 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);
    }

    /*
    --
    2.19.1
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