timers: Fix data race in timer_stats_account_timer()

timer_stats_account_timer() reads timer->start_site, then checks it
for NULL and then re-reads it again, while
timer_stats_timer_clear_start_info() can concurrently reset
timer->start_site to NULL. This should not lead to crashes, but can
double number of entries in timer stats as start_site is used during
comparison, the doubled entries will have unuseful NULL start_site.

Read timer->start_site only once in timer_stats_account_timer().

The data race was found with KernelThreadSanitizer (KTSAN).

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Cc: andreyknvl@google.com
Cc: glider@google.com
Cc: kcc@google.com
Cc: ktsan@googlegroups.com
Cc: john.stultz@linaro.org
Link: http://lkml.kernel.org/r/1442584463-69553-1-git-send-email-dvyukov@google.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/time/timer.c

@@ -461,10 +461,17 @@ void __timer_stats_timer_set_start_info(struct timer_list *timer, void *addr)
 
 static void timer_stats_account_timer(struct timer_list *timer)
 {
-	if (likely(!timer->start_site))
+	void *site;
+
+	/*
+	 * start_site can be concurrently reset by
+	 * timer_stats_timer_clear_start_info()
+	 */
+	site = READ_ONCE(timer->start_site);
+	if (likely(!site))
 		return;
 
-	timer_stats_update_stats(timer, timer->start_pid, timer->start_site,
+	timer_stats_update_stats(timer, timer->start_pid, site,
 				 timer->function, timer->start_comm,
 				 timer->flags);
 }