Message-ID: <3b4ef4687af55b2332fd128ff3f3f810f5681df0.camel@mailbox.org>
Date: Wed, 23 Jul 2025 08:56:01 +0200
From: Philipp Stanner <phasta@...lbox.org>
To: Matthew Brost <matthew.brost@...el.com>, phasta@...nel.org
Cc: Danilo Krummrich <dakr@...nel.org>, James Flowers
 <bold.zone2373@...tmail.com>, ckoenig.leichtzumerken@...il.com, 
 maarten.lankhorst@...ux.intel.com, mripard@...nel.org, tzimmermann@...e.de,
  airlied@...il.com, simona@...ll.ch, skhan@...uxfoundation.org, 
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
 linux-kernel-mentees@...ts.linux.dev, Tvrtko Ursulin
 <tvrtko.ursulin@...lia.com>
Subject: Re: [PATCH] drm/sched: Prevent stopped entities from being added to
 the run queue.

On Tue, 2025-07-22 at 01:45 -0700, Matthew Brost wrote:
> On Tue, Jul 22, 2025 at 01:07:29AM -0700, Matthew Brost wrote:
> > On Tue, Jul 22, 2025 at 09:37:11AM +0200, Philipp Stanner wrote:
> > > On Mon, 2025-07-21 at 11:07 -0700, Matthew Brost wrote:
> > > > On Mon, Jul 21, 2025 at 12:14:31PM +0200, Danilo Krummrich wrote:
> > > > > On Mon Jul 21, 2025 at 10:16 AM CEST, Philipp Stanner wrote:
> > > > > > On Mon, 2025-07-21 at 09:52 +0200, Philipp Stanner wrote:
> > > > > > > On Sun, 2025-07-20 at 16:56 -0700, James Flowers wrote:
> > > > > > > > diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> > > > > > > > index bfea608a7106..997a2cc1a635 100644
> > > > > > > > --- a/drivers/gpu/drm/scheduler/sched_main.c
> > > > > > > > +++ b/drivers/gpu/drm/scheduler/sched_main.c
> > > > > > > > @@ -172,8 +172,10 @@ void drm_sched_rq_update_fifo_locked(struct drm_sched_entity *entity,
> > > > > > > >  
> > > > > > > >  	entity->oldest_job_waiting = ts;
> > > > > > > >  
> > > > > > > > -	rb_add_cached(&entity->rb_tree_node, &rq->rb_tree_root,
> > > > > > > > -		      drm_sched_entity_compare_before);
> > > > > > > > +	if (!entity->stopped) {
> > > > > > > > +		rb_add_cached(&entity->rb_tree_node, &rq->rb_tree_root,
> > > > > > > > +			      drm_sched_entity_compare_before);
> > > > > > > > +	}
> > > > > > > 
> > > > > > > If this is a race, then this patch here is broken, too, because you're
> > > > > > > checking the 'stopped' boolean as the callers of that function do, too
> > > > > > > – just later. :O
> > > > > > > 
> > > > > > > Could still race, just less likely.
> > > > > > > 
> > > > > > > The proper way to fix it would then be to address the issue where the
> > > > > > > locking is supposed to happen. Let's look at, for example,
> > > > > > > drm_sched_entity_push_job():
> > > > > > > 
> > > > > > > 
> > > > > > > void drm_sched_entity_push_job(struct drm_sched_job *sched_job)
> > > > > > > {
> > > > > > > 	(Bla bla bla)
> > > > > > > 
> > > > > > >  	…………
> > > > > > > 
> > > > > > > 	/* first job wakes up scheduler */
> > > > > > > 	if (first) {
> > > > > > > 		struct drm_gpu_scheduler *sched;
> > > > > > > 		struct drm_sched_rq *rq;
> > > > > > > 
> > > > > > > 		/* Add the entity to the run queue */
> > > > > > > 		spin_lock(&entity->lock);
> > > > > > > 		if (entity->stopped) {                  <---- Aha!
> > > > > > > 			spin_unlock(&entity->lock);
> > > > > > > 
> > > > > > > 			DRM_ERROR("Trying to push to a killed entity\n");
> > > > > > > 			return;
> > > > > > > 		}
> > > > > > > 
> > > > > > > 		rq = entity->rq;
> > > > > > > 		sched = rq->sched;
> > > > > > > 
> > > > > > > 		spin_lock(&rq->lock);
> > > > > > > 		drm_sched_rq_add_entity(rq, entity);
> > > > > > > 
> > > > > > > 		if (drm_sched_policy == DRM_SCHED_POLICY_FIFO)
> > > > > > > 			drm_sched_rq_update_fifo_locked(entity, rq, submit_ts); <---- bumm!
> > > > > > > 
> > > > > > > 		spin_unlock(&rq->lock);
> > > > > > > 		spin_unlock(&entity->lock);
> > > > > > > 
> > > > > > > But the locks are still being hold. So that "shouldn't be happening"(tm).
> > > > > > > 
> > > > > > > Interesting. AFAICS only drm_sched_entity_kill() and drm_sched_fini()
> > > > > > > stop entities. The former holds appropriate locks, but drm_sched_fini()
> > > > > > > doesn't. So that looks like a hot candidate to me. Opinions?
> > > > > > > 
> > > > > > > On the other hand, aren't drivers prohibited from calling
> > > > > > > drm_sched_entity_push_job() after calling drm_sched_fini()? If the
> > > > > > > fuzzer does that, then it's not the scheduler's fault.
> > > > > 
> > > > > Exactly, this is the first question to ask.
> > > > > 
> > > > > And I think it's even more restrictive:
> > > > > 
> > > > > In drm_sched_fini()
> > > > > 
> > > > > 	for (i = DRM_SCHED_PRIORITY_KERNEL; i < sched->num_rqs; i++) {
> > > > > 		struct drm_sched_rq *rq = sched->sched_rq[i];
> > > > > 
> > > > > 		spin_lock(&rq->lock);
> > > > > 		list_for_each_entry(s_entity, &rq->entities, list)
> > > > > 			/*
> > > > > 			 * Prevents reinsertion and marks job_queue as idle,
> > > > > 			 * it will be removed from the rq in drm_sched_entity_fini()
> > > > > 			 * eventually
> > > > > 			 */
> > > > > 			s_entity->stopped = true;
> > > > > 		spin_unlock(&rq->lock);
> > > > > 		kfree(sched->sched_rq[i]);
> > > > > 	}
> > > > > 
> > > > > In drm_sched_entity_kill()
> > > > > 
> > > > > 	static void drm_sched_entity_kill(struct drm_sched_entity *entity)
> > > > > 	{
> > > > > 		struct drm_sched_job *job;
> > > > > 		struct dma_fence *prev;
> > > > > 
> > > > > 		if (!entity->rq)
> > > > > 			return;
> > > > > 
> > > > > 		spin_lock(&entity->lock);
> > > > > 		entity->stopped = true;
> > > > > 		drm_sched_rq_remove_entity(entity->rq, entity);
> > > > > 		spin_unlock(&entity->lock);
> > > > > 
> > > > > 		[...]
> > > > > 	}
> > > > > 
> > > > > If this runs concurrently, this is a UAF as well.
> > > > > 
> > > > > Personally, I have always been working with the assupmtion that entites have to
> > > > > be torn down *before* the scheduler, but those lifetimes are not documented
> > > > > properly.
> > > > 
> > > > Yes, this is my assumption too. I would even take it further: an entity
> > > > shouldn't be torn down until all jobs associated with it are freed as
> > > > well. I think this would solve a lot of issues I've seen on the list
> > > > related to UAF, teardown, etc.
> > > 
> > > That's kind of impossible with the new tear down design, because
> > > drm_sched_fini() ensures that all jobs are freed on teardown. And
> > > drm_sched_fini() wouldn't be called before all jobs are gone,
> > > effectively resulting in a chicken-egg-problem, or rather: the driver
> > > implementing its own solution for teardown.
> > > 
> > 
> > I've read this four times and I'm still generally confused.
> > 
> > "drm_sched_fini ensures that all jobs are freed on teardown" — Yes,
> > that's how a refcounting-based solution works. drm_sched_fini would
> > never be called if there were pending jobs.
> > 
> > "drm_sched_fini() wouldn't be called before all jobs are gone" — See
> > above.
> > 
> > "effectively resulting in a chicken-and-egg problem" — A job is created
> > after the scheduler, and it holds a reference to the scheduler until
> > it's freed. I don't see how this idiom applies.
> > 
> > "the driver implementing its own solution for teardown" — It’s just
> > following the basic lifetime rules I outlined below. Perhaps Xe was
> > ahead of its time, but the number of DRM scheduler blowups we've had is
> > zero — maybe a strong indication that this design is correct.
> > 
> 
> Sorry—self-reply.
> 
> To expand on this: the reason Xe implemented a refcount-based teardown
> solution is because the internals of the DRM scheduler during teardown
> looked wildly scary. A lower layer should not impose its will on upper
> layers. I think that’s the root cause of all the problems I've listed.
> 
> In my opinion, we should document the lifetime rules I’ve outlined, fix
> all drivers accordingly, and assert these rules in the scheduler layer.


Everyone had a separate solution for that. Nouveau used a waitqueue.
That's what happens when there's no centralized mechanism for solving a
problem.

Did you see the series we recently merged which repairs the memory
leaks of drm/sched? It had been around for quite some time.

https://lore.kernel.org/dri-devel/20250701132142.76899-3-phasta@kernel.org/


P.

> 
> Matt
> 
> > Matt
> > 
> > > P.
> > > 
> > > 
> > > > 
> > > > > 
> > > > > There are two solutions:
> > > > > 
> > > > >   (1) Strictly require all entities to be torn down before drm_sched_fini(),
> > > > >       i.e. stick to the natural ownership and lifetime rules here (see below).
> > > > > 
> > > > >   (2) Actually protect *any* changes of the relevent fields of the entity
> > > > >       structure with the entity lock.
> > > > > 
> > > > > While (2) seems rather obvious, we run into lock inversion with this approach,
> > > > > as you note below as well. And I think drm_sched_fini() should not mess with
> > > > > entities anyways.
> > > > > 
> > > > > The ownership here seems obvious:
> > > > > 
> > > > > The scheduler *owns* a resource that is used by entities. Consequently, entities
> > > > > are not allowed to out-live the scheduler.
> > > > > 
> > > > > Surely, the current implementation to just take the resource away from the
> > > > > entity under the hood can work as well with appropriate locking, but that's a
> > > > > mess.
> > > > > 
> > > > > If the resource *really* needs to be shared for some reason (which I don't see),
> > > > > shared ownership, i.e. reference counting, is much less error prone.
> > > > 
> > > > Yes, Xe solves all of this via reference counting (jobs refcount the
> > > > entity). It's a bit easier in Xe since the scheduler and entities are
> > > > the same object due to their 1:1 relationship. But even in non-1:1
> > > > relationships, an entity could refcount the scheduler. The teardown
> > > > sequence would then be: all jobs complete on the entity → teardown the
> > > > entity → all entities torn down → teardown the scheduler.
> > > > 
> > > > Matt
> > > 

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