On Wed, May 23, 2018 at 1:50 AM, Joel Fernandes (Google) <[email protected]> wrote: > Currently there is a race in schedutil code for slow-switch single-CPU > systems. Fix it by enforcing ordering the write to work_in_progress to > happen before the read of next_freq. > > Kthread Sched update > > sugov_work() sugov_update_single() > > lock(); > // The CPU is free to rearrange below > // two in any order, so it may clear > // the flag first and then read next > // freq. Lets assume it does. > work_in_progress = false > > if (work_in_progress) > return; > > sg_policy->next_freq = 0; > freq = sg_policy->next_freq; > sg_policy->next_freq = > real-freq; > unlock(); > > Reported-by: Viresh Kumar <[email protected]> > CC: Rafael J. Wysocki <[email protected]> > CC: Peter Zijlstra <[email protected]> > CC: Ingo Molnar <[email protected]> > CC: Patrick Bellasi <[email protected]> > CC: Juri Lelli <[email protected]> > Cc: Luca Abeni <[email protected]> > CC: Todd Kjos <[email protected]> > CC: [email protected] > CC: [email protected] > CC: [email protected] > Signed-off-by: Joel Fernandes (Google) <[email protected]> > --- > I split this into separate patch, because this race can also happen in > mainline. > > kernel/sched/cpufreq_schedutil.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/kernel/sched/cpufreq_schedutil.c > b/kernel/sched/cpufreq_schedutil.c > index 5c482ec38610..ce7749da7a44 100644 > --- a/kernel/sched/cpufreq_schedutil.c > +++ b/kernel/sched/cpufreq_schedutil.c > @@ -401,6 +401,13 @@ static void sugov_work(struct kthread_work *work) > */ > raw_spin_lock_irqsave(&sg_policy->update_lock, flags); > freq = sg_policy->next_freq; > + > + /* > + * sugov_update_single can access work_in_progress without > update_lock, > + * make sure next_freq is read before work_in_progress is set. > + */ > + smp_mb(); > +
This requires a corresponding barrier somewhere else. > sg_policy->work_in_progress = false; > raw_spin_unlock_irqrestore(&sg_policy->update_lock, flags); > > -- Also, as I said I actually would prefer to use the spinlock in the one-CPU case when the kthread is used. I'll have a patch for that shortly.
