Am 08.07.21 um 23:54 schrieb Daniel Vetter:
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)
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
drivers/gpu/drm/scheduler/sched_entity.c | 27 ++++++++++++++++++++++-- 1 file changed, 25 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c index 64d398166644..6366006c0fcf 100644 --- a/drivers/gpu/drm/scheduler/sched_entity.c +++ b/drivers/gpu/drm/scheduler/sched_entity.c @@ -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();
That whole stuff needs to be inside the spsc queue, not outside.
Otherwise drm_sched_entity_is_idle() won't work either and cause a lot of trouble during process tear down.
Christian.
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;- /* queue non-empty, stay on the same engine */
- if (spsc_queue_count(&entity->job_queue)) return;
- fence = READ_ONCE(entity->last_scheduled);
- /*
* 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;