[PATCH 3/4] ARM: imx6q: 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 imx6q_enter_wait() is beyond the last generic
need_resched() check and it performs a wfi right away after the call to
rcu_idle_enter(). 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: 1a67b9263e06 (ARM: imx6q: Fixup RCU usage for cpuidle)
Cc: stable@xxxxxxxxxxxxxxx
Cc: Shawn Guo <shawnguo@xxxxxxxxxx>
Cc: Sascha Hauer <s.hauer@xxxxxxxxxxxxxx>
Cc: Pengutronix Kernel Team <kernel@xxxxxxxxxxxxxx>
Cc: Fabio Estevam <festevam@xxxxxxxxx>
Cc: NXP Linux Team <linux-imx@xxxxxxx>
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>
---
 arch/arm/mach-imx/cpuidle-imx6q.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/arm/mach-imx/cpuidle-imx6q.c b/arch/arm/mach-imx/cpuidle-imx6q.c
index 094337dc1bc7..31a60d257d3d 100644
--- a/arch/arm/mach-imx/cpuidle-imx6q.c
+++ b/arch/arm/mach-imx/cpuidle-imx6q.c
@@ -25,7 +25,12 @@ static int imx6q_enter_wait(struct cpuidle_device *dev,
 	raw_spin_unlock(&cpuidle_lock);
 
 	rcu_idle_enter();
-	cpu_do_idle();
+	/*
+	 * Last need_resched() check must come after rcu_idle_enter()
+	 * which may wake up RCU internal tasks.
+	 */
+	if (!need_resched())
+		cpu_do_idle();
 	rcu_idle_exit();
 
 	raw_spin_lock(&cpuidle_lock);
-- 
2.25.1