o enter dyntick-idle mode despite having RCU callbacks pending, it posts a timer on that CPU to force a wakeup on that CPU. This wakeup ensures that the CPU will eventually handle the end of the grace period, including invoking its RCU callbacks. However, Pascal Chapperon's test setup shows that the timer handler rcu_idle_gp_timer_func() really does get invoked in some cases. This is problematic because this can cause the CPU that entered dyntick-idle mode despite still having RCU callbacks pending to remain in dyntick-idle mode indefinitely, which means that its RCU callbacks might never be invoked. This situation can result in grace-period delays or even system hangs, which matches Pascal's observations of slow boot-up and shutdown (https://lkml.org/lkml/2012/4/5/142). See also the bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=806548 This commit therefore causes the "should never be invoked" timer handler rcu_idle_gp_timer_func() to use smp_call_function_single() to wake up the CPU for which the timer was intended, allowing that CPU to invoke its RCU callbacks in a timely manner. Reported-by: Pascal Chapperon Signed-off-by: Paul E. McKenney Signed-off-by: Paul E. McKenney hFDx