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Date:   Tue, 22 Dec 2020 02:37:11 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Fabio Estevam <festevam@...il.com>, stable@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        NXP Linux Team <linux-imx@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>
Subject: [PATCH 3/4] ARM: imx6q: Fix missing need_resched() check after rcu_idle_enter()

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@...nel.org>
Fixes: 1a67b9263e06 (ARM: imx6q: Fixup RCU usage for cpuidle)
Cc: stable@...r.kernel.org
Cc: Shawn Guo <shawnguo@...nel.org>
Cc: Sascha Hauer <s.hauer@...gutronix.de>
Cc: Pengutronix Kernel Team <kernel@...gutronix.de>
Cc: Fabio Estevam <festevam@...il.com>
Cc: NXP Linux Team <linux-imx@....com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
 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

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