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

["Paul E. McKenney" <paulmck@kernel.org>]

On Tue, Dec 22, 2020 at 02:37:09AM +0100, 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 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@kernel.org>
> Fixes: 96d3fd0d315a (rcu: Break call_rcu() deadlock involving scheduler and perf)
> Cc: stable@vger.kernel.org
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar<mingo@kernel.org>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Tested-by: Paul E. McKenney <paulmck@kernel.org>

> ---
>  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
>