hrtimer: Protect lockless access to timer->base

The update to timer->base is protected by the base->cpu_base->lock().
However, hrtimer_cancel_wait_running() does access it lockless.  So the
compiler is allowed to refetch timer->base which can cause havoc when the
timer base is changed concurrently.

Use READ_ONCE() to prevent this.

[ tglx: Adapted from a RT patch ]
Signed-off-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190821092409.13225-2-julien.grall@arm.com

kernel/time/hrtimer.c

@@ -1214,7 +1214,8 @@ static void hrtimer_sync_wait_running(struct hrtimer_cpu_base *cpu_base,
  */
 void hrtimer_cancel_wait_running(const struct hrtimer *timer)
 {
-	struct hrtimer_clock_base *base = timer->base;
+	/* Lockless read. Prevent the compiler from reloading it below */
+	struct hrtimer_clock_base *base = READ_ONCE(timer->base);
 
 	if (!timer->is_soft || !base || !base->cpu_base) {
 		cpu_relax();