Messages in this thread Patch in this message | | | Date | Tue, 17 Nov 2020 10:46:21 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched: Fix data-race in wakeup |
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On Tue, Nov 17, 2020 at 10:29:36AM +0100, Peter Zijlstra wrote: > 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: > > > /* 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()?
How's this then? It still doesn't explicitly call out the specific race, but does mention the more fundamental issue that wakelist queueing doesn't respect the regular rules anymore.
--- 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 +784,21 @@ struct task_struct { /* Unserialized, strictly 'current' */ + /* + * This field must not be in the scheduler word above due to wakelist + * queueing no longer being serialized by p->on_cpu. However: + * + * p->XXX = X; 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 = Y; + * + * guarantees all stores of 'current' are visible before + * ->sched_remote_wakeup gets used, so it can be in this word. + */ + unsigned sched_remote_wakeup:1; + /* Bit to tell LSMs we're in execve(): */ unsigned in_execve:1; unsigned in_iowait:1;
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