On Tue, Nov 17, 2020 at 09:15:46AM +0000, Will Deacon wrote:
> On Tue, Nov 17, 2020 at 09:30:16AM +0100, Peter Zijlstra wrote:
> > Subject: sched: Fix data-race in wakeup
> > From: Peter Zijlstra <pet...@infradead.org>
> > Date: Tue Nov 17 09:08:41 CET 2020
> >
> > Mel reported that on some ARM64 platforms loadavg goes bananas and
> > tracked it down to the following data race:
> >
> > CPU0 CPU1
> >
> > schedule()
> > prev->sched_contributes_to_load = X;
> > deactivate_task(prev);
> >
> > try_to_wake_up()
> > if (p->on_rq &&) // false
> > if (smp_load_acquire(&p->on_cpu) &&
> > // true
> > ttwu_queue_wakelist())
> > p->sched_remote_wakeup = Y;
> >
> > smp_store_release(prev->on_cpu, 0);
>
> (nit: I suggested this race over at [1] ;)
Ah, I'll ammend and get you a Debugged-by line or something ;-)
> > where both p->sched_contributes_to_load and p->sched_remote_wakeup are
> > in the same word, and thus the stores X and Y race (and can clobber
> > one another's data).
> >
> > Whereas prior to commit c6e7bd7afaeb ("sched/core: Optimize ttwu()
> > spinning on p->on_cpu") the p->on_cpu handoff serialized access to
> > p->sched_remote_wakeup (just as it still does with
> > p->sched_contributes_to_load) that commit broke that by calling
> > ttwu_queue_wakelist() with p->on_cpu != 0.
> >
> > However, due to
> >
> > p->XXX ttwu()
> > schedule() if (p->on_rq && ...) // false
> > smp_mb__after_spinlock() if (smp_load_acquire(&p->on_cpu) &&
> > deactivate_task() ttwu_queue_wakelist())
> > p->on_rq = 0; p->sched_remote_wakeup = X;
> >
> > We can be sure any 'current' store is complete and 'current' is
> > guaranteed asleep. Therefore we can move p->sched_remote_wakeup into
> > the current flags word.
> >
> > Note: while the observed failure was loadavg accounting gone wrong due
> > to ttwu() cobbering p->sched_contributes_to_load, the reverse problem
> > is also possible where schedule() clobbers p->sched_remote_wakeup,
> > this could result in enqueue_entity() wrecking ->vruntime and causing
> > scheduling artifacts.
> >
> > Fixes: c6e7bd7afaeb ("sched/core: Optimize ttwu() spinning on p->on_cpu")
> > Reported-by: Mel Gorman <mgor...@techsingularity.net>
> > Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
> > ---
> > include/linux/sched.h | 13 ++++++++++++-
> > 1 file changed, 12 insertions(+), 1 deletion(-)
> >
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -775,7 +775,6 @@ struct task_struct {
> > unsigned sched_reset_on_fork:1;
> > unsigned sched_contributes_to_load:1;
> > unsigned sched_migrated:1;
> > - unsigned sched_remote_wakeup:1;
> > #ifdef CONFIG_PSI
> > unsigned sched_psi_wake_requeue:1;
> > #endif
> > @@ -785,6 +784,18 @@ struct task_struct {
> >
> > /* Unserialized, strictly 'current' */
> >
> > + /*
> > + * p->in_iowait = 1; ttwu()
> > + * schedule() if (p->on_rq && ..) // false
> > + * smp_mb__after_spinlock(); if (smp_load_acquire(&p->on_cpu) &&
> > //true
> > + * deactivate_task() ttwu_queue_wakelist())
> > + * p->on_rq = 0; p->sched_remote_wakeup = X;
> > + *
> > + * Guarantees all stores of 'current' are visible before
> > + * ->sched_remote_wakeup gets used.
>
> I'm still not sure this is particularly clear -- don't we want to highlight
> that the store of p->on_rq is unordered wrt the update to
> p->sched_contributes_to_load() in deactivate_task()?
I can explicitly call that out I suppose.
> I dislike bitfields with a passion, but the fix looks good:
I don't particularly hate them, they're just a flag field with names on
(in this case).
> Acked-by: Will Deacon <w...@kernel.org>
Thanks!
> Now the million dollar question is why KCSAN hasn't run into this. Hrmph.
kernel/sched/Makefile:KCSAN_SANITIZE := n
might have something to do with that, I suppose.
