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

[Mika Kuoppala]

Chris Wilson <chris at chris-wilson.co.uk> writes:

> 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.

There is chicken and egg problem here that I fail to grasp.

If the retire work is the mechanism which triggers the request
freeing, then the order should be fine. As the engine is still
there for last request.

Or is the problem that it happens inside the worker which is inside
the engine?

-Mika

>
> 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);
>  }
> -- 
> 2.25.0
>
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