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    Subject[PATCH AUTOSEL for 4.14 061/100] sched/wait: Fix add_wait_queue() behavioral change
    Date
    From: Omar Sandoval <osandov@fb.com>

    [ Upstream commit c6b9d9a33029014446bd9ed84c1688f6d3d4eab9 ]

    The following cleanup commit:

    50816c48997a ("sched/wait: Standardize internal naming of wait-queue entries")

    ... unintentionally changed the behavior of add_wait_queue() from
    inserting the wait entry at the head of the wait queue to the tail
    of the wait queue.

    Beyond a negative performance impact this change in behavior
    theoretically also breaks wait queues which mix exclusive and
    non-exclusive waiters, as non-exclusive waiters will not be
    woken up if they are queued behind enough exclusive waiters.

    Signed-off-by: Omar Sandoval <osandov@fb.com>
    Reviewed-by: Jens Axboe <axboe@kernel.dk>
    Acked-by: Peter Zijlstra <peterz@infradead.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: kernel-team@fb.com
    Fixes: ("sched/wait: Standardize internal naming of wait-queue entries")
    Link: http://lkml.kernel.org/r/a16c8ccffd39bd08fdaa45a5192294c784b803a7.1512544324.git.osandov@fb.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
    ---
    kernel/sched/wait.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
    index 98feab7933c7..929ecb7d6b78 100644
    --- a/kernel/sched/wait.c
    +++ b/kernel/sched/wait.c
    @@ -27,7 +27,7 @@ void add_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq

    wq_entry->flags &= ~WQ_FLAG_EXCLUSIVE;
    spin_lock_irqsave(&wq_head->lock, flags);
    - __add_wait_queue_entry_tail(wq_head, wq_entry);
    + __add_wait_queue(wq_head, wq_entry);
    spin_unlock_irqrestore(&wq_head->lock, flags);
    }
    EXPORT_SYMBOL(add_wait_queue);
    --
    2.11.0
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