[PATCH 1/4] sched/idle: Fix missing need_resched() check after rcu_idle_enter()

[Frederic Weisbecker]

Entering RCU idle mode may cause a deferred wake up of an RCU NOCB_GP
kthread (rcuog) to be serviced.

Usually a wake up happening while running the idle task is spotted in
one of the need_resched() checks carefully placed within the idle loop
that can break to the scheduler.

Unfortunately in default_idle_call(), the call to rcu_idle_enter() is
already beyond the last need_resched() check and we may halt the CPU
with a resched request unhandled, leaving the task hanging.

Fix this with performing a last minute need_resched() check after
calling rcu_idle_enter().

Reported-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Fixes: 96d3fd0d315a (rcu: Break call_rcu() deadlock involving scheduler and perf)
Cc: stable@xxxxxxxxxxxxxxx
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar<mingo@xxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
---
 kernel/sched/idle.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 305727ea0677..1af60dc50beb 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -109,15 +109,21 @@ void __cpuidle default_idle_call(void)
 		rcu_idle_enter();
 		lockdep_hardirqs_on(_THIS_IP_);
 
-		arch_cpu_idle();
+		/*
+		 * Last need_resched() check must come after rcu_idle_enter()
+		 * which may wake up RCU internal tasks.
+		 */
+		if (!need_resched()) {
+			arch_cpu_idle();
+			raw_local_irq_disable();
+		}
 
 		/*
-		 * OK, so IRQs are enabled here, but RCU needs them disabled to
-		 * turn itself back on.. funny thing is that disabling IRQs
-		 * will cause tracing, which needs RCU. Jump through hoops to
-		 * make it 'work'.
+		 * OK, so IRQs are enabled after arch_cpu_idle(), but RCU needs
+		 * them disabled to turn itself back on.. funny thing is that
+		 * disabling IRQs will cause tracing, which needs RCU. Jump through
+		 * hoops to make it 'work'.
 		 */
-		raw_local_irq_disable();
 		lockdep_hardirqs_off(_THIS_IP_);
 		rcu_idle_exit();
 		lockdep_hardirqs_on(_THIS_IP_);
-- 
2.25.1