[PATCH v2] drm/scheduler: fix timeout worker setup for out of order job completions

[Lucas Stach]

Am Montag, den 06.08.2018, 14:57 +0200 schrieb Christian König:
> Am 03.08.2018 um 16:29 schrieb Lucas Stach:
> > drm_sched_job_finish() is a work item scheduled for each finished job on
> > a unbound system workqueue. This means the workers can execute out of order
> > with regard to the real hardware job completions.
> > 
> > If this happens queueing a timeout worker for the first job on the ring
> > mirror list is wrong, as this may be a job which has already finished
> > executing. Fix this by reorganizing the code to always queue the worker
> > for the next job on the list, if this job hasn't finished yet. This is
> > robust against a potential reordering of the finish workers.
> > 
> > Also move out the timeout worker cancelling, so that we don't need to
> > take the job list lock twice. As a small optimization list_del is used
> > to remove the job from the ring mirror list, as there is no need to
> > reinit the list head in the job we are about to free.
> > 
> > > > Signed-off-by: Lucas Stach <l.stach at pengutronix.de>
> > ---
> > v2: - properly handle last job in the ring
> >      - check correct fence for compeletion
> > ---
> >   drivers/gpu/drm/scheduler/gpu_scheduler.c | 22 ++++++++++------------
> >   1 file changed, 10 insertions(+), 12 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/scheduler/gpu_scheduler.c b/drivers/gpu/drm/scheduler/gpu_scheduler.c
> > index 44d480768dfe..574875e2c206 100644
> > --- a/drivers/gpu/drm/scheduler/gpu_scheduler.c
> > +++ b/drivers/gpu/drm/scheduler/gpu_scheduler.c
> > @@ -452,24 +452,22 @@ static void drm_sched_job_finish(struct work_struct *work)
> > > >   						   finish_work);
> > > >   	struct drm_gpu_scheduler *sched = s_job->sched;
> >   
> > > > -	/* remove job from ring_mirror_list */
> > > > -	spin_lock(&sched->job_list_lock);
> > > > -	list_del_init(&s_job->node);
> > > > -	if (sched->timeout != MAX_SCHEDULE_TIMEOUT) {
> > > > -		struct drm_sched_job *next;
> > -
> > > > -		spin_unlock(&sched->job_list_lock);
> > > > +	if (sched->timeout != MAX_SCHEDULE_TIMEOUT)
> >   		cancel_delayed_work_sync(&s_job->work_tdr);
> 
> That is unfortunately still racy here.
> 
> Between canceling the job and removing it from the list someone could 
> actually start the time (in theory) :)
> 
> Cancel it, remove it from the list and cancel it again.

I don't see how. If we end up in this worker the finished fence of the
job is already certainly signaled (as this is what triggers queueing of
the worker). So even if some other worker manages to find this job as
the next job in the list, the dma_fence_is_signaled check should
prevent the timeout worker from getting scheduled again.

> BTW: You could completely drop the "if (sched->timeout != 
> MAX_SCHEDULE_TIMEOUT)" here cause canceling is harmless as long as the 
> structure is initialized.

Right.

Regards,
Lucas

> Christian.
> 
> > > > -		spin_lock(&sched->job_list_lock);
> >   
> > > > -		/* queue TDR for next job */
> > > > -		next = list_first_entry_or_null(&sched->ring_mirror_list,
> > > > -						struct drm_sched_job, node);
> > > > +	spin_lock(&sched->job_list_lock);
> > > > +	/* queue TDR for next job */
> > > > +	if (sched->timeout != MAX_SCHEDULE_TIMEOUT &&
> > > > +	    !list_is_last(&s_job->node, &sched->ring_mirror_list)) {
> > > > +		struct drm_sched_job *next = list_next_entry(s_job, node);
> >   
> > > > -		if (next)
> > > > +		if (!dma_fence_is_signaled(&next->s_fence->finished))
> > > >   			schedule_delayed_work(&next->work_tdr, sched->timeout);
> > > >   	}
> > > > +	/* remove job from ring_mirror_list */
> > > > +	list_del(&s_job->node);
> > > >   	spin_unlock(&sched->job_list_lock);
> > +
> > > >   	dma_fence_put(&s_job->s_fence->finished);
> > > >   	sched->ops->free_job(s_job);
> >   }
> 
>