Commit 19d9f422 authored Apr 14, 2015 by Thomas Gleixner

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@linutronix.de>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Preeti U Murthy <preeti@linux.vnet.ibm.com>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/r/20150414203503.580273114@linutronix.de


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

parent 61699e13

kernel/time/hrtimer.c

+9 −0

Original line number	Original line	Diff line number	Diff line
	@@ -991,6 +991,15 @@ int hrtimer_try_to_cancel(struct hrtimer *timer)
	unsigned long flags;		unsigned long flags;
	int ret = -1;		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);		base = lock_hrtimer_base(timer, &flags);

	if (!hrtimer_callback_running(timer))		if (!hrtimer_callback_running(timer))