rcu: Add warning for RCU_FAST_NO_HZ timer firing

RCU_FAST_NO_HZ uses a timer to limit the time that a CPU with callbacks
can remain in dyntick-idle mode.  This timer is cancelled when the CPU
exits idle, and therefore should never fire.  However, if the timer
were migrated to some other CPU for whatever reason (1) the timer could
actually fire and (2) firing on some other CPU would fail to wake up the
CPU with callbacks, possibly resulting in sluggishness or a system hang.

This commit therfore adds a WARN_ON_ONCE() to the timer handler in order
to detect this condition.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 50c17975d4f42904384225b37b1478d41d1e8fa5..ad61da79b311e131c159723d2003a833a4325dd6 100644 (file)
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -2065,6 +2065,7 @@ static bool rcu_cpu_has_nonlazy_callbacks(int cpu)
  */
 static void rcu_idle_gp_timer_func(unsigned long unused)
 {
+       WARN_ON_ONCE(1); /* Getting here can hang the system... */
        trace_rcu_prep_idle("Timer");
 }