[Intel-gfx] [PATCH v2 4/5] drm/i915/guc: Make adding GuC work items lockless

[Chris Wilson]

Quoting Michał Winiarski (2017-09-12 18:40:55)
> @@ -434,11 +430,16 @@ static void guc_wq_item_append(struct i915_guc_client *client,
>          */
>         BUILD_BUG_ON(wqi_size != 16);
>  
> -       /* postincrement WQ tail for next time */
> -       wq_off = client->wq_tail;
> +       /* Find our offset and postincrement WQ tail for next time, free space
> +        * is guaranteed.
> +        */
> +       do {
> +               wq_off = READ_ONCE(desc->tail);
> +               wq_next = (wq_off + wqi_size) & (GUC_WQ_SIZE - 1);
> +               GEM_BUG_ON(CIRC_SPACE(wq_off, READ_ONCE(desc->head),
> +                                     GUC_WQ_SIZE) < wqi_size);
> +       } while (cmpxchg(&desc->tail, wq_off, wq_next) != wq_off);
>         GEM_BUG_ON(wq_off & (wqi_size - 1));

I forgot the trick in cmpxchg to avoid the READ_ONCE in every loop:
	wq_next = READ_ONCE(desc->tail);
	do {
		wq_off = wq_next;
		wq_next = (wq_off + wqi_size) & (GUC_WQ_SIZE - 1);
		GEM_BUG_ON(...);
	} while ((wq_next = cmpxchg(&desc->tail, wq_off, wq_next) != wq_off);
-Chris