[tip:locking/core] locking/mutex: Allow next waiter lockless wakeup

From: tip-bot for Davidlohr Bueso
Date: Mon Feb 29 2016 - 06:22:58 EST

Commit-ID: 1329ce6fbbe4536592dfcfc8d64d61bfeb598fe6
Gitweb: http://git.kernel.org/tip/1329ce6fbbe4536592dfcfc8d64d61bfeb598fe6
Author: Davidlohr Bueso <dave@xxxxxxxxxxxx>
AuthorDate: Sun, 24 Jan 2016 18:23:43 -0800
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Mon, 29 Feb 2016 10:02:42 +0100

locking/mutex: Allow next waiter lockless wakeup

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@xxxxxxx>
Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Ding Tianhong <dingtianhong@xxxxxxxxxx>
Cc: Jason Low <jason.low2@xxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: Waiman Long <waiman.long@xxxxxxx>
Cc: Will Deacon <Will.Deacon@xxxxxxx>
Link: http://lkml.kernel.org/r/20160125022343.GA3322@xxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
kernel/locking/mutex.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 0551c21..e364b42 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -716,6 +716,7 @@ 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
@@ -743,11 +744,11 @@ __mutex_unlock_common_slowpath(struct mutex *lock, int nested)
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);