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

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Date:	Mon, 29 Feb 2016 03:21:47 -0800
From:	tip-bot for Davidlohr Bueso <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	Will.Deacon@....com, mingo@...nel.org, paulmck@...ibm.com,
	linux-kernel@...r.kernel.org, Waiman.Long@....com,
	paulmck@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
	dingtianhong@...wei.com, waiman.long@....com,
	tim.c.chen@...ux.intel.com, dave@...olabs.net, dbueso@...e.de,
	peterz@...radead.org, torvalds@...ux-foundation.org,
	jason.low2@...com, hpa@...or.com, tglx@...utronix.de
Subject: [tip:locking/core] locking/mutex: Allow next waiter lockless wakeup

Commit-ID:  1329ce6fbbe4536592dfcfc8d64d61bfeb598fe6
Gitweb:     http://git.kernel.org/tip/1329ce6fbbe4536592dfcfc8d64d61bfeb598fe6
Author:     Davidlohr Bueso <dave@...olabs.net>
AuthorDate: Sun, 24 Jan 2016 18:23:43 -0800
Committer:  Ingo Molnar <mingo@...nel.org>
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@....com>
Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Ding Tianhong <dingtianhong@...wei.com>
Cc: Jason Low <jason.low2@...com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Paul E. McKenney <paulmck@...ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Tim Chen <tim.c.chen@...ux.intel.com>
Cc: Waiman Long <waiman.long@....com>
Cc: Will Deacon <Will.Deacon@....com>
Link: http://lkml.kernel.org/r/20160125022343.GA3322@linux-uzut.site
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 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);
 }
 
 /*