linux-kernel - [tip:timers/core] hrtimer: Avoid locking in hrtimer

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Date:	Wed, 22 Apr 2015 12:14:23 -0700
From:	tip-bot for Thomas Gleixner <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	viresh.kumar@...aro.org, preeti@...ux.vnet.ibm.com,
	fweisbec@...il.com, tglx@...utronix.de, mingo@...nel.org,
	hpa@...or.com, peterz@...radead.org, mtosatti@...hat.com,
	linux-kernel@...r.kernel.org
Subject: [tip:timers/core] hrtimer: Avoid locking in hrtimer_cancel()
  if timer not active

Commit-ID:  19d9f4225dd6a47fca430f15eeae345ceb95c301
Gitweb:     http://git.kernel.org/tip/19d9f4225dd6a47fca430f15eeae345ceb95c301
Author:     Thomas Gleixner <tglx@...utronix.de>
AuthorDate: Tue, 14 Apr 2015 21:09:25 +0000
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Wed, 22 Apr 2015 17:06:52 +0200

hrtimer: Avoid locking in hrtimer_cancel() if timer not active

We can do a lockless check for hrtimer_active before actually taking
the lock in hrtimer[_try_to]_cancel. This is useful for hotpath users
like nanosleep as they avoid the lock dance when the timer has
expired.

This is safe because active is true when the timer is enqueued or the
callback is running. Taking the hrtimer base lock does not protect
against concurrent hrtimer_start calls, the callsite has to do the
proper serialization itself.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Acked-by: Peter Zijlstra <peterz@...radead.org>
Cc: Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc: Viresh Kumar <viresh.kumar@...aro.org>
Cc: Marcelo Tosatti <mtosatti@...hat.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Link: http://lkml.kernel.org/r/20150414203503.580273114@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/time/hrtimer.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index beab02d..3bac942 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -991,6 +991,15 @@ int hrtimer_try_to_cancel(struct hrtimer *timer)
 	unsigned long flags;
 	int ret = -1;
 
+	/*
+	 * Check lockless first. If the timer is not active (neither
+	 * enqueued nor running the callback, nothing to do here.  The
+	 * base lock does not serialize against a concurrent enqueue,
+	 * so we can avoid taking it.
+	 */
+	if (!hrtimer_active(timer))
+		return 0;
+
 	base = lock_hrtimer_base(timer, &flags);
 
 	if (!hrtimer_callback_running(timer))
--
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