[PATCH] SWDEV-197284 - drm/amdgpu: Only use the peek function in productor side is not correct

[Christian König]

Am 12.08.19 um 09:42 schrieb Emily Deng:
> For spsc queue, use peek function would cause error in productor side.
> As for the last element, when the push job is occurring during pop job, the peek function
> will not be updated in time, and it will return NULL.
>
> So use queue count for double confirming the job queue is empty.

For the upstream branch I'm going to push my patch which is not as 
invasive as this one.

Christian.

>
> Signed-off-by: Emily Deng <Emily.Deng at amd.com>
> ---
>   drivers/gpu/drm/scheduler/sched_entity.c | 4 ++--
>   include/drm/spsc_queue.h                 | 7 +++----
>   2 files changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c
> index 35ddbec..e74894f 100644
> --- a/drivers/gpu/drm/scheduler/sched_entity.c
> +++ b/drivers/gpu/drm/scheduler/sched_entity.c
> @@ -95,7 +95,7 @@ static bool drm_sched_entity_is_idle(struct drm_sched_entity *entity)
>   	rmb(); /* for list_empty to work without lock */
>   
>   	if (list_empty(&entity->list) ||
> -	    spsc_queue_peek(&entity->job_queue) == NULL)
> +	    ((spsc_queue_peek(&entity->job_queue) == NULL) && !spsc_queue_count(&entity->job_queue)))
>   		return true;
>   
>   	return false;
> @@ -281,7 +281,7 @@ void drm_sched_entity_fini(struct drm_sched_entity *entity)
>   	/* Consumption of existing IBs wasn't completed. Forcefully
>   	 * remove them here.
>   	 */
> -	if (spsc_queue_peek(&entity->job_queue)) {
> +	if ((spsc_queue_peek(&entity->job_queue) == NULL) && !spsc_queue_count(&entity->job_queue)) {
>   		if (sched) {
>   			/* Park the kernel for a moment to make sure it isn't processing
>   			 * our enity.
> diff --git a/include/drm/spsc_queue.h b/include/drm/spsc_queue.h
> index 125f096..78092b9 100644
> --- a/include/drm/spsc_queue.h
> +++ b/include/drm/spsc_queue.h
> @@ -54,9 +54,8 @@ static inline void spsc_queue_init(struct spsc_queue *queue)
>   
>   static inline struct spsc_node *spsc_queue_peek(struct spsc_queue *queue)
>   {
> -	return queue->head;
> +	return READ_ONCE(queue->head);
>   }
> -
>   static inline int spsc_queue_count(struct spsc_queue *queue)
>   {
>   	return atomic_read(&queue->job_count);
> @@ -70,9 +69,9 @@ static inline bool spsc_queue_push(struct spsc_queue *queue, struct spsc_node *n
>   
>   	preempt_disable();
>   
> +	atomic_inc(&queue->job_count);
>   	tail = (struct spsc_node **)atomic_long_xchg(&queue->tail, (long)&node->next);
>   	WRITE_ONCE(*tail, node);
> -	atomic_inc(&queue->job_count);
>   
>   	/*
>   	 * In case of first element verify new node will be visible to the consumer
> @@ -93,6 +92,7 @@ static inline struct spsc_node *spsc_queue_pop(struct spsc_queue *queue)
>   	/* Verify reading from memory and not the cache */
>   	smp_rmb();
>   
> +	atomic_dec(&queue->job_count);
>   	node = READ_ONCE(queue->head);
>   
>   	if (!node)
> @@ -113,7 +113,6 @@ static inline struct spsc_node *spsc_queue_pop(struct spsc_queue *queue)
>   		}
>   	}
>   
> -	atomic_dec(&queue->job_count);
>   	return node;
>   }
>