Re: [Intel-xe] [PATCH v6 5/7] drm/sched: Split free_job into own work item

[Boris Brezillon <boris.brezillon@collabora.com>]

On Mon, 23 Oct 2023 14:16:06 +0200
Boris Brezillon <boris.brezillon@collabora.com> wrote:

> Hi,
> 
> On Tue, 17 Oct 2023 08:09:56 -0700
> Matthew Brost <matthew.brost@intel.com> wrote:
> 
> > +static void drm_sched_run_job_work(struct work_struct *w)
> > +{
> > +	struct drm_gpu_scheduler *sched =
> > +		container_of(w, struct drm_gpu_scheduler, work_run_job);
> > +	struct drm_sched_entity *entity;
> > +	struct dma_fence *fence;
> > +	struct drm_sched_fence *s_fence;
> > +	struct drm_sched_job *sched_job;
> > +	int r;
> >  
> > -		atomic_inc(&sched->hw_rq_count);
> > -		drm_sched_job_begin(sched_job);
> > +	if (READ_ONCE(sched->pause_submit))
> > +		return;
> > +
> > +	entity = drm_sched_select_entity(sched, true);
> > +	if (!entity)
> > +		return;
> >  
> > -		trace_drm_run_job(sched_job, entity);
> > -		fence = sched->ops->run_job(sched_job);
> > +	sched_job = drm_sched_entity_pop_job(entity);
> > +	if (!sched_job) {
> >  		complete_all(&entity->entity_idle);
> > -		drm_sched_fence_scheduled(s_fence, fence);
> > +		return;	/* No more work */
> > +	}
> >  
> > -		if (!IS_ERR_OR_NULL(fence)) {
> > -			/* Drop for original kref_init of the fence */
> > -			dma_fence_put(fence);
> > +	s_fence = sched_job->s_fence;
> >  
> > -			r = dma_fence_add_callback(fence, &sched_job->cb,
> > -						   drm_sched_job_done_cb);
> > -			if (r == -ENOENT)
> > -				drm_sched_job_done(sched_job, fence->error);
> > -			else if (r)
> > -				DRM_DEV_ERROR(sched->dev, "fence add callback failed (%d)\n",
> > -					  r);
> > -		} else {
> > -			drm_sched_job_done(sched_job, IS_ERR(fence) ?
> > -					   PTR_ERR(fence) : 0);
> > -		}
> > +	atomic_inc(&sched->hw_rq_count);
> > +	drm_sched_job_begin(sched_job);
> >  
> > -		wake_up(&sched->job_scheduled);
> > +	trace_drm_run_job(sched_job, entity);
> > +	fence = sched->ops->run_job(sched_job);
> > +	complete_all(&entity->entity_idle);
> > +	drm_sched_fence_scheduled(s_fence, fence);
> > +
> > +	if (!IS_ERR_OR_NULL(fence)) {
> > +		/* Drop for original kref_init of the fence */
> > +		dma_fence_put(fence);
> > +
> > +		r = dma_fence_add_callback(fence, &sched_job->cb,
> > +					   drm_sched_job_done_cb);
> > +		if (r == -ENOENT)
> > +			drm_sched_job_done(sched_job, fence->error);
> > +		else if (r)
> > +			DRM_DEV_ERROR(sched->dev, "fence add callback failed (%d)\n", r);
> > +	} else {
> > +		drm_sched_job_done(sched_job, IS_ERR(fence) ?
> > +				   PTR_ERR(fence) : 0);
> >  	}  
> 
> Just ran into a race condition when using a non-ordered workqueue
> for drm_sched:
> 
> thread A							thread B
> 
> drm_sched_run_job_work()			
>   drm_sched_job_begin()
>     // inserts jobA in pending_list
> 
> 								drm_sched_free_job_work()
> 								  drm_sched_get_cleanup_job()
> 								    // check first job in pending list
> 								    // if s_fence->parent == NULL, consider
> 								    // for cleanup
> 								  ->free_job(jobA)  
> 								    drm_sched_job_cleanup()
> 								      // set sched_job->s_fence = NULL
> 
>     ->run_job()  
>     drm_sched_fence_scheduled()

Correction: the NULL pointer deref happens in drm_sched_job_done()
(when the driver returns an error directly) not in
drm_sched_fence_scheduled(), but the problem remains the same.


>       // sched_job->s_fence->parent = parent_fence
>       // BOOM => NULL pointer deref
> 
> For now, I'll just use a dedicated ordered wq, but if we claim
> multi-threaded workqueues are supported, this is probably worth fixing.
> I know there's been some discussions about when the timeout should be
> started, and the job insertion in the pending_list is kinda related.
> If we want this insertion to happen before ->run_job() is called, we
> need a way to flag when a job is inserted, but not fully submitted yet.
> 
> Regards,
> 
> Boris