Re: [RFC][PATCH] sched/ext: Avoid null ptr traversal when ->put_prev_task() is called with NULL next

[Kuba Piecuch]

Hello John,

On Sat Dec 6, 2025 at 2:22 AM UTC, John Stultz wrote:
> I can't say I understand why this doesn't trip usually without
> proxy-exec. And in newer kernels there are way fewer
> put_prev_task(), and I can't easily reproduce the issue now
> even with proxy-exec.

That's probably because put_prev_task_scx() with next == NULL is always
preceded by a dequeue, clearing SCX_TASK_QUEUED from p->scx.flags, so we don't
reach the problematic sched_class_above() check because it only happens when
the flag is set.

> But we still have one put_prev_task() call left in core.c that
> seems like it could trip this, so I wanted to send this out for
> consideration.

I'm assuming you're referring to the one in sched_change_begin().
It looks like it's impossible for an outside observer holding a CPU's rq lock
to observe a task that is running on that CPU and isn't queued, i.e.
'running' implies 'queued' (I'm new to the scheduler so I may be wrong here).
That would explain why dequeue_task() is always called before put_prev_task().
Does proxy execution break that assumption?

Best,
Kuba