[PATCH] sched: Fix data-race in wakeup

[Peter Zijlstra]

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;