Re: [PATCH 2/2] drm/sched: serialize job_timeout and scheduler

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Aug 31, 2021 at 02:59:02PM +0200, Daniel Vetter wrote:
> Can we please have some actual commit message here, with detailed
> explanation of the race/bug/whatever, how you fix it and why this is the
> best option?
> 
> On Tue, Aug 31, 2021 at 06:35:39PM +0800, Monk Liu wrote:
> > tested-by: jingwen chen <jingwen.chen@amd.com>
> > Signed-off-by: Monk Liu <Monk.Liu@amd.com>
> > Signed-off-by: jingwen chen <jingwen.chen@amd.com>
> > ---
> >  drivers/gpu/drm/scheduler/sched_main.c | 24 ++++--------------------
> >  1 file changed, 4 insertions(+), 20 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> > index ecf8140..894fdb24 100644
> > --- a/drivers/gpu/drm/scheduler/sched_main.c
> > +++ b/drivers/gpu/drm/scheduler/sched_main.c
> > @@ -319,19 +319,17 @@ static void drm_sched_job_timedout(struct work_struct *work)
> >  	sched = container_of(work, struct drm_gpu_scheduler, work_tdr.work);
> >  
> >  	/* Protects against concurrent deletion in drm_sched_get_cleanup_job */
> > +	if (!__kthread_should_park(sched->thread))
> 
> This is a __ function, i.e. considered internal, and it's lockless atomic,
> i.e. unordered. And you're not explaining why this works.
> 
> Iow it's probably buggy, and an just unconditionally parking the kthread
> is probably the right thing to do. If it's not the right thing to do,
> there's a bug here for sure.

Also why don't we reuse the function drivers already have to stop a
scheduler thread? We seem to have two kthread_park now, that's probably
one too much.
-Daniel

> > +		kthread_park(sched->thread);
> > +
> >  	spin_lock(&sched->job_list_lock);
> >  	job = list_first_entry_or_null(&sched->pending_list,
> >  				       struct drm_sched_job, list);
> >  
> >  	if (job) {
> > -		/*
> > -		 * Remove the bad job so it cannot be freed by concurrent
> > -		 * drm_sched_cleanup_jobs. It will be reinserted back after sched->thread
> > -		 * is parked at which point it's safe.
> > -		 */
> > -		list_del_init(&job->list);
> >  		spin_unlock(&sched->job_list_lock);
> >  
> > +		/* vendor's timeout_job should call drm_sched_start() */
> >  		status = job->sched->ops->timedout_job(job);
> >  
> >  		/*
> > @@ -393,20 +391,6 @@ void drm_sched_stop(struct drm_gpu_scheduler *sched, struct drm_sched_job *bad)
> >  	kthread_park(sched->thread);
> >  
> >  	/*
> > -	 * Reinsert back the bad job here - now it's safe as
> > -	 * drm_sched_get_cleanup_job cannot race against us and release the
> > -	 * bad job at this point - we parked (waited for) any in progress
> > -	 * (earlier) cleanups and drm_sched_get_cleanup_job will not be called
> > -	 * now until the scheduler thread is unparked.
> > -	 */
> > -	if (bad && bad->sched == sched)
> > -		/*
> > -		 * Add at the head of the queue to reflect it was the earliest
> > -		 * job extracted.
> > -		 */
> > -		list_add(&bad->list, &sched->pending_list);
> > -
> > -	/*
> >  	 * Iterate the job list from later to  earlier one and either deactive
> >  	 * their HW callbacks or remove them from pending list if they already
> >  	 * signaled.
> > -- 
> > 2.7.4
> > 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch