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>

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index beab02d3ff1e5a3d8907d139e9a0a39624273ff5..3bac94269a981113a3475b2ba34ae4b46a52d934 100644 (file)
--- 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))