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;
