[Intel-gfx] [PATCH 1/4] drm/i915/gt: Prevent queuing retire workers on the virtual engine

[Tvrtko Ursulin]

On 06/02/2020 20:49, Chris Wilson wrote:
> Virtual engines are fleeting. They carry a reference count and may be freed
> when their last request is retired. This makes them unsuitable for the
> task of housing engine->retire.work so assert that it is not used.
> 
> Tvrtko tracked down an instance where we did indeed violate this rule.
> In virtual_submit_request, we flush a completed request directly with
> __i915_request_submit and this causes us to queue that request on the
> veng's breadcrumb list and signal it. Leading us down a path where we
> should not attach the retire.
> 
> Reported-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Fixes: dc93c9b69315 ("drm/i915/gt: Schedule request retirement when signaler idles")
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> ---
>   drivers/gpu/drm/i915/gt/intel_breadcrumbs.c | 3 +++
>   drivers/gpu/drm/i915/gt/intel_gt_requests.c | 3 +++
>   2 files changed, 6 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> index 0ba524a414c6..cbad7fe722ce 100644
> --- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> +++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> @@ -136,6 +136,9 @@ static void add_retire(struct intel_breadcrumbs *b, struct intel_timeline *tl)
>   	struct intel_engine_cs *engine =
>   		container_of(b, struct intel_engine_cs, breadcrumbs);
>   
> +	if (unlikely(intel_engine_is_virtual(engine)))
> +		engine = intel_virtual_engine_get_sibling(engine, 0);
> +
>   	intel_engine_add_retire(engine, tl);
>   }
>   
> diff --git a/drivers/gpu/drm/i915/gt/intel_gt_requests.c b/drivers/gpu/drm/i915/gt/intel_gt_requests.c
> index 7ef1d37970f6..8a5054f21bf8 100644
> --- a/drivers/gpu/drm/i915/gt/intel_gt_requests.c
> +++ b/drivers/gpu/drm/i915/gt/intel_gt_requests.c
> @@ -99,6 +99,9 @@ static bool add_retire(struct intel_engine_cs *engine,
>   void intel_engine_add_retire(struct intel_engine_cs *engine,
>   			     struct intel_timeline *tl)
>   {
> +	/* We don't deal well with the engine disappearing beneath us */
> +	GEM_BUG_ON(intel_engine_is_virtual(engine));
> +
>   	if (add_retire(engine, tl))
>   		schedule_work(&engine->retire_work);
>   }
> 

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>

Regards,

Tvrtko