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 <[email protected]> > Fixes: 96d3fd0d315a (rcu: Break call_rcu() deadlock involving scheduler and > perf) > Cc: [email protected] > Cc: Peter Zijlstra <[email protected]> > Cc: Rafael J. Wysocki <[email protected]> > Cc: Thomas Gleixner <[email protected]> > Cc: Ingo Molnar<[email protected]> > Signed-off-by: Frederic Weisbecker <[email protected]>
Tested-by: Paul E. McKenney <[email protected]> > --- > 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 >
