[Intel-gfx] [PATCH] drm/i915: Force ordering on request submission and hangcheck

[Chris Wilson]

On Wed, Jan 13, 2016 at 07:04:53PM +0200, Mika Kuoppala wrote:
> Hangcheck is run on irq context and might be active on a

s/irq/process/ that we pretend is irq-like.

> completely different CPU that is submitting requests. And as
> we have been very careful not to add locking to hangcheck to guard
> against driver failures, we need to be careful with the coherency.
> 
> Update ring last seqno and add to request lists early, with
> write memory barrier, before pushing the request to ring. On
> hangcheck side, use read memory barrier before inspecting the ring
> last seqno. This ensures that when hangcheck is inspecting the
> state, it will not see a active ring without requests in it,
> and then falsely report it as a missed irq situation.

You are just moving the race, not closing it afaict.

>  ring_idle(struct intel_engine_cs *ring, u32 seqno)
>  {
> -	return (list_empty(&ring->request_list) ||
> -		i915_seqno_passed(seqno, ring->last_submitted_seqno));
> +	rmb(); /* Ordering against __i915_add_request() */
> +	return (i915_seqno_passed(seqno, ring->last_submitted_seqno)
> +		|| list_empty(&ring->request_list));

But we can get rid of list_empty()!

If we just embrace the race, we can do this simply by checking for the
same idle waiter twice.

http://lists.freedesktop.org/archives/intel-gfx/2016-January/084514.html

which is the most recent variant.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre