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

[Daniel Vetter]

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 at amd.com>
> Signed-off-by: Monk Liu <Monk.Liu at amd.com>
> Signed-off-by: jingwen chen <jingwen.chen at 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.
-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