Re: [PATCH 08/10] dma-buf/resv: replace shared fence with new fences container

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Christian König (2019-08-21 13:31:45)
> @@ -117,17 +120,10 @@ i915_gem_busy_ioctl(struct drm_device *dev, void *data,
>                 busy_check_writer(rcu_dereference(obj->base.resv->fence_excl));
>  
>         /* Translate shared fences to READ set of engines */
> -       list = rcu_dereference(obj->base.resv->fence);
> -       if (list) {
> -               unsigned int shared_count = list->shared_count, i;
> -
> -               for (i = 0; i < shared_count; ++i) {
> -                       struct dma_fence *fence =
> -                               rcu_dereference(list->shared[i]);
> -
> -                       args->busy |= busy_check_reader(fence);
> -               }
> -       }
> +       readers = dma_resv_fences_get_rcu(&obj->base.resv->readers);
> +       dma_fence_array_for_each(fence, cursor, readers)
> +               args->busy |= busy_check_reader(fence);
> +       dma_fence_put(readers);

That's underwhelming, the full-mb shows up in scaling tests (I'll test
the impact of this series later). Something like,

do {
	read = 0;
	fences = dma_resv_fences_get_deref(&obj->base.resv->readers);
	dma_fence_array_for_each(fence, cursor, fences)
		read |= busy_check_reader(fence);
	smp_rmb();
} while (dma_resv_fences_get_deref(obj->readers) != fences)

do {
	fences = dma_resv_fences_get_deref(&obj->base.resv->fences);
	write = busy_check_writer(fences);
	smp_rmb();
} while (dma_resv_fences_get_deref(obj->writes) != fences)

args->busy = write | read;

Perhaps?
-Chris