Re: [PATCH v4 5/6] drm/sched: Use parent fence instead of finished

[Christian König]

Am 29.09.22 um 16:53 schrieb Steven Price:
> On 14/09/2022 17:43, Arvind Yadav wrote:
> > Using the parent fence instead of the finished fence
> > to get the job status. This change is to avoid GPU
> > scheduler timeout error which can cause GPU reset.
> I'm able to reproduce crashes on Panfrost and I believe this commit is
> the cause. Specifically it's possible for job->s_fence->parent to be NULL.
>
> The underlying issue seems to involve drm_sched_resubmit_jobs_ext() - if
> the run_jobs() callback returns an error it will set s_fence->parent to
> NULL after signalling s_fence->finished:
>
> > 		fence = sched->ops->run_job(s_job);
> > 		i++;
> >
> > 		if (IS_ERR_OR_NULL(fence)) {
> > 			if (IS_ERR(fence))
> > 				dma_fence_set_error(&s_fence->finished, PTR_ERR(fence));
> >
> > 			s_job->s_fence->parent = NULL;
> I don't understand the reasoning behind this change, but it doesn't seem
> right to be using the parent fence when we have code which can be
> setting that pointer to NULL.
>
> Since I don't understand the reasoning my only suggestion is to revert
> this patch (and potentially the dependent patch "dma-buf: Check status
> of enable-signaling bit on debug"?).
>
> Can anyone suggest a better fix?

Well, first of all please absolutely don't use 
drm_sched_resubmit_jobs_ext()!

It was an extremely bad idea in amdgpu to approach GPU by re-submitting 
jobs and it was an even worse idea to push this into the scheduler.

The design of dma_fence is that you submit that once and *only* once and 
then get a result for this submission. If re-submission is desirable it 
should be done in userspace or at least higher levels.

Apart from that, yes a NULL check is missing here but that should be 
trivial to fix.

Thanks,
Christian.

> Thanks,
>
> Steve
>
> > Signed-off-by: Arvind Yadav <Arvind.Yadav@xxxxxxx>
> > Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@xxxxxxx>
> > ---
> >
> > changes in v1,v2 - Enable signaling for finished fence in sche_main()
> > is removed
> >
> > ---
> >   drivers/gpu/drm/scheduler/sched_main.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> > index e0ab14e0fb6b..2ac28ad11432 100644
> > --- a/drivers/gpu/drm/scheduler/sched_main.c
> > +++ b/drivers/gpu/drm/scheduler/sched_main.c
> > @@ -829,7 +829,7 @@ drm_sched_get_cleanup_job(struct drm_gpu_scheduler *sched)
> >   	job = list_first_entry_or_null(&sched->pending_list,
> >   				       struct drm_sched_job, list);
> >   
> > -	if (job && dma_fence_is_signaled(&job->s_fence->finished)) {
> > +	if (job && dma_fence_is_signaled(job->s_fence->parent)) {
> >   		/* remove job from pending_list */
> >   		list_del_init(&job->list);
> >   
> > @@ -841,7 +841,7 @@ drm_sched_get_cleanup_job(struct drm_gpu_scheduler *sched)
> >   
> >   		if (next) {
> >   			next->s_fence->scheduled.timestamp =
> > -				job->s_fence->finished.timestamp;
> > +				job->s_fence->parent->timestamp;
> >   			/* start TO timer for next job */
> >   			drm_sched_start_timeout(sched);
> >   		}