[PATCH tip/core/rcu 13/14] rcu: Handle NOCB callbacks from irq-disabled idle code

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

If an RCU callback is queued on a no-CBs CPU from idle code with irqs
disabled, and if that CPU stays idle forever after, the callback will
never be invoked.  This commit therefore adds a check for this situation
in ____call_rcu_nocb(), invoking the RCU core solely for the purpose
of the ensuing return-to-idle transition.  (If the CPU doesn't return
to idle, the next scheduling-clock interrupt will fix things up.)

Reported-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree_plugin.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 304f42c6543b..700e5b37bf26 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2129,6 +2129,17 @@ static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp,
 		trace_rcu_callback(rdp->rsp->name, rhp,
 				   -atomic_long_read(&rdp->nocb_q_count_lazy),
 				   -atomic_long_read(&rdp->nocb_q_count));
+
+	/*
+	 * If called from an extended quiescent state with interrupts
+	 * disabled, invoke the RCU core in order to allow the idle-entry
+	 * deferred-wakeup check to function.
+	 */
+	if (irqs_disabled_flags(flags) &&
+	    !rcu_is_watching() &&
+	    cpu_online(smp_processor_id()))
+		invoke_rcu_core();
+
 	return true;
 }
 
-- 
1.8.1.5