[Intel-gfx] [PATCH 2/6] drm/i915: Reset ring space estimate after unwinding the request

[Chris Wilson]

Quoting Chris Wilson (2018-03-07 13:42:22)
> With a series of unusual events (a sequence of interrupted request
> allocations), we could gradually leak the ring->space estimate by
> unwinding the ring back to the start of the request, but not return the
> used space back to the ring. Eventually and with great misfortune, it
> would be possible to trigger ring->space exhaustion with no requests on
> the ring.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/i915_request.c     | 1 +
>  drivers/gpu/drm/i915/intel_ringbuffer.c | 1 +
>  2 files changed, 2 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> index d437beac3969..efa9ee557f31 100644
> --- a/drivers/gpu/drm/i915/i915_request.c
> +++ b/drivers/gpu/drm/i915/i915_request.c
> @@ -798,6 +798,7 @@ i915_request_alloc(struct intel_engine_cs *engine, struct i915_gem_context *ctx)
>  
>  err_unwind:
>         rq->ring->emit = rq->head;
> +       intel_ring_update_space(rq->ring);

Ok, skip this one as we will correct ourselves next time we
wait_for_space. It's just the next one where we weren't maintaining
ring->tail that was the issue.
-Chris