Messages in this thread | | | Date | Tue, 17 Nov 2020 09:15:46 +0000 | From | Will Deacon <> | Subject | Re: [PATCH] sched: Fix data-race in wakeup |
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On Tue, Nov 17, 2020 at 09:30:16AM +0100, Peter Zijlstra wrote: > On Mon, Nov 16, 2020 at 07:31:49PM +0000, Mel Gorman wrote: > > > And this works. > > Yay! > > > sched_psi_wake_requeue can probably stay with the other three fields > > given they are under the rq lock but sched_remote_wakeup needs to move > > out. > > I _think_ we can move the bit into the unserialized section below. > > It's a bit cheecky, but it should work I think because the only time we > actually use this bit, we're guaranteed the task isn't actually running, > so current doesn't exist. > > I suppose the question is wether this is worth saving 31 bits over... > > How's this? > > --- > Subject: sched: Fix data-race in wakeup > From: Peter Zijlstra <peterz@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] ;)
> 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 <mgorman@techsingularity.net> > Signed-off-by: Peter Zijlstra (Intel) <peterz@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 dislike bitfields with a passion, but the fix looks good:
Acked-by: Will Deacon <will@kernel.org>
Now the million dollar question is why KCSAN hasn't run into this. Hrmph.
Will
[1] https://lore.kernel.org/r/20201116131102.GA29992@willie-the-truck
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