rcu: Eliminate RCU_FAST_NO_HZ grace-period hang

With the new implementation of RCU_FAST_NO_HZ, it was possible to hang
RCU grace periods as follows:

o	CPU 0 attempts to go idle, cycles several times through the
	rcu_prepare_for_idle() loop, then goes dyntick-idle when
	RCU needs nothing more from it, while still having at least
	on RCU callback pending.

o	CPU 1 goes idle with no callbacks.

Both CPUs can then stay in dyntick-idle mode indefinitely, preventing
the RCU grace period from ever completing, possibly hanging the system.

This commit therefore prevents CPUs that have RCU callbacks from entering
dyntick-idle mode.  This approach also eliminates the need for the
end-of-grace-period IPIs used previously.

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

1 files changed, 0 insertions, 2 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 7fb8b0e60811..13fab4a9f9fb 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1086,7 +1086,6 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
          * callbacks are waiting on the grace period that just now
          * completed.
          */
-        rcu_schedule_wake_gp_end();
         if (*rdp->nxttail[RCU_WAIT_TAIL] == NULL) {
                 raw_spin_unlock(&rnp->lock);     /* irqs remain disabled. */
 
@@ -1672,7 +1671,6 @@ static void rcu_process_callbacks(struct softirq_action *unused)
                                 &__get_cpu_var(rcu_sched_data));
         __rcu_process_callbacks(&rcu_bh_state, &__get_cpu_var(rcu_bh_data));
         rcu_preempt_process_callbacks();
-        rcu_wake_cpus_for_gp_end();
         trace_rcu_utilization("End RCU core");
 }