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.
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..4e1124ed80e0 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_job_arm() 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; + + /* queue non-empty, stay on the same engine */ + if (spsc_queue_count(&entity->job_queue)) return;
- fence = READ_ONCE(entity->last_scheduled); + fence = entity->last_scheduled; + + /* + * Only when the queue is empty are we guaranteed the 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(); + + /* stay on the same engine if the previous job hasn't finished */ if (fence && !dma_fence_is_signaled(fence)) return;