Re: [PATCH 3/4] ARM: imx6q: Fix missing need_resched() check after rcu_idle_enter()
Hi Frederic, On Mon, Jan 4, 2021 at 12:21 PM Frederic Weisbecker wrote: > > 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(). Shouldn't tif_need_resched() be used instead of need_resched() in the commit log?
[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 Reviewed-by: Rafael J. Wysocki Fixes: 1a67b9263e06 (ARM: imx6q: Fixup RCU usage for cpuidle) Cc: sta...@vger.kernel.org Cc: Shawn Guo Cc: Sascha Hauer Cc: Pengutronix Kernel Team Cc: Fabio Estevam Cc: NXP Linux Team Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: Ingo Molnar Signed-off-by: Frederic Weisbecker --- arch/arm/mach-imx/cpuidle-imx6q.c | 8 +++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/arch/arm/mach-imx/cpuidle-imx6q.c b/arch/arm/mach-imx/cpuidle-imx6q.c index 094337dc1bc7..1115f4dc6d1d 100644 --- a/arch/arm/mach-imx/cpuidle-imx6q.c +++ b/arch/arm/mach-imx/cpuidle-imx6q.c @@ -5,6 +5,7 @@ #include #include +#include #include #include @@ -25,7 +26,12 @@ 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 (!tif_need_resched()) + cpu_do_idle(); rcu_idle_exit(); raw_spin_lock(&cpuidle_lock); -- 2.25.1
[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 Fixes: 1a67b9263e06 (ARM: imx6q: Fixup RCU usage for cpuidle) Cc: sta...@vger.kernel.org Cc: Shawn Guo Cc: Sascha Hauer Cc: Pengutronix Kernel Team Cc: Fabio Estevam Cc: NXP Linux Team Cc: Peter Zijlstra Cc: Rafael J. Wysocki Cc: Thomas Gleixner Cc: Ingo Molnar Signed-off-by: Frederic Weisbecker --- 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
