回复: [PATCH] sched_ext: Fix NULL pointer dereferences in put_prev_task_scx

[liuwenfang]

Thanks for your feedback.
This is triggered in kernel modules developed In the mobile scenario: 
tasks on rq are migrated while the current cpu should be halted for power saving policy.
Its migration logic:
drain_rq_cpu_stop -- migrate_all_tasks :
for (;;) {
	if (rq->nr_running == 1)
		break;
	for_each_class(class) {
		next = class->pick_next_task(rq);
		if (next) {
			next->sched_class->put_prev_task(rq, next, NULL);
			break;
		}
	}
	if (is_idle_task(next))
		break;
	dest_cpu = select_task_rq(next...);
	move_queued_task(rq, rf, next, dest_cpu);
	...
}

put_prev_task in this function is selected to update util and statistics info for each runnable tasks,
here they are not dequeued yet.

Best regards,
 
> On Mon, Jun 09, 2025 at 11:36:15AM +0000, liuwenfang wrote:
> > As put_prev_task can be used in other kernel modules which can lead to
> > a NULL pointer. Fix this by checking for a valid next.
> 
> Actually, put_prev_task() should be used only within kernel/sched/ and, in theory,
> you should have done a dequeue_task() before put_prev_task() in this scenario,
> so SCX_TASK_QUEUED shouldn't be set in p->scx.flags.
> 
> The change might still make sense, but can you clarify how you triggered the
> NULL pointer dereference?
> 
> Thanks,
> -Andrea
> 
> >
> > Signed-off-by: l00013971 <l00013971@xxxxxxxxxxx>
> > ---
> >  kernel/sched/ext.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index
> > f5133249f..6a579babd 100644
> > --- a/kernel/sched/ext.c
> > +++ b/kernel/sched/ext.c
> > @@ -3262,7 +3262,7 @@ static void put_prev_task_scx(struct rq *rq, struct
> task_struct *p,
> >  		 * ops.enqueue() that @p is the only one available for this cpu,
> >  		 * which should trigger an explicit follow-up scheduling event.
> >  		 */
> > -		if (sched_class_above(&ext_sched_class, next->sched_class)) {
> > +		if (next && sched_class_above(&ext_sched_class, next->sched_class))
> > +{
> >  			WARN_ON_ONCE(!static_branch_unlikely(&scx_ops_enq_last));
> >  			do_enqueue_task(rq, p, SCX_ENQ_LAST, -1);
> >  		} else {
> > --
> > 2.17.1