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    Subject[PATCH 4.4 32/87] locking/mutex: Allow next waiter lockless wakeup
    Date
    4.4-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Davidlohr Bueso <dave@stgolabs.net>

    commit 1329ce6fbbe4536592dfcfc8d64d61bfeb598fe6 upstream.

    Make use of wake-queues and enable the wakeup to occur after releasing the
    wait_lock. This is similar to what we do with rtmutex top waiter,
    slightly shortening the critical region and allow other waiters to
    acquire the wait_lock sooner. In low contention cases it can also help
    the recently woken waiter to find the wait_lock available (fastpath)
    when it continues execution.

    Reviewed-by: Waiman Long <Waiman.Long@hpe.com>
    Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Ding Tianhong <dingtianhong@huawei.com>
    Cc: Jason Low <jason.low2@hp.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Paul E. McKenney <paulmck@us.ibm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Tim Chen <tim.c.chen@linux.intel.com>
    Cc: Waiman Long <waiman.long@hpe.com>
    Cc: Will Deacon <Will.Deacon@arm.com>
    Link: http://lkml.kernel.org/r/20160125022343.GA3322@linux-uzut.site
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    kernel/locking/mutex.c | 5 +++--
    1 file changed, 3 insertions(+), 2 deletions(-)

    --- a/kernel/locking/mutex.c
    +++ b/kernel/locking/mutex.c
    @@ -719,6 +719,7 @@ static inline void
    __mutex_unlock_common_slowpath(struct mutex *lock, int nested)
    {
    unsigned long flags;
    + WAKE_Q(wake_q);

    /*
    * As a performance measurement, release the lock before doing other
    @@ -746,11 +747,11 @@ __mutex_unlock_common_slowpath(struct mu
    struct mutex_waiter, list);

    debug_mutex_wake_waiter(lock, waiter);
    -
    - wake_up_process(waiter->task);
    + wake_q_add(&wake_q, waiter->task);
    }

    spin_unlock_mutex(&lock->wait_lock, flags);
    + wake_up_q(&wake_q);
    }

    /*

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