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drm/sched: Barriers are needed for entity->last_scheduled

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It might be good enough on x86 with just READ_ONCE, but the write side
should then at least be WRITE_ONCE because x86 has total store order.

It's definitely not enough on arm.

Fix this proplery, which means
- explain the need for the barrier in both places
- point at the other side in each comment

Also pull out the !sched_list case as the first check, so that the
code flow is clearer.

While at it sprinkle some comments around because it was very
non-obvious to me what's actually going on here and why.

Note that we really need full barriers here, at first I thought
store-release and load-acquire on ->last_scheduled would be enough,
but we actually requiring ordering between that and the queue state.

v2: Put smp_rmp() in the right place and fix up comment (Andrey)

Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Steven Price <steven.price@arm.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210805104705.862416-4-daniel.vetter@ffwll.ch
This commit is contained in:
Daniel Vetter 2021-08-05 12:46:48 +02:00
parent 357285a2d1
commit b0a5303d4e
1 changed files with 25 additions and 2 deletions
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27
drivers/gpu/drm/scheduler/sched_entity.c
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@ -439,8 +439,16 @@ struct drm_sched_job *drm_sched_entity_pop_job(struct drm_sched_entity *entity)
dma_fence_set_error(&sched_job->s_fence->finished, -ECANCELED);
dma_fence_put(entity->last_scheduled);
entity->last_scheduled = dma_fence_get(&sched_job->s_fence->finished);
/*
* If the queue is empty we allow drm_sched_entity_select_rq() to
* locklessly access ->last_scheduled. This only works if we set the
* pointer before we dequeue and if we a write barrier here.
*/
smp_wmb();
spsc_queue_pop(&entity->job_queue);
return sched_job;
}
@ -459,10 +467,25 @@ void drm_sched_entity_select_rq(struct drm_sched_entity *entity)
struct drm_gpu_scheduler *sched;
struct drm_sched_rq *rq;
if (spsc_queue_count(&entity->job_queue) || !entity->sched_list)
/* single possible engine and already selected */
if (!entity->sched_list)
return;
fence = READ_ONCE(entity->last_scheduled);
/* queue non-empty, stay on the same engine */
if (spsc_queue_count(&entity->job_queue))
return;
/*
* Only when the queue is empty are we guaranteed that the scheduler
* thread cannot change ->last_scheduled. To enforce ordering we need
* a read barrier here. See drm_sched_entity_pop_job() for the other
* side.
*/
smp_rmb();
fence = entity->last_scheduled;
/* stay on the same engine if the previous job hasn't finished */
if (fence && !dma_fence_is_signaled(fence))
return;