[Intel-gfx] [PATCH 01/32] drm/i915: Seal races between async GPU cancellation, retirement and signaling

[Chris Wilson]

Quoting Tvrtko Ursulin (2019-04-18 11:32:31)
> 
> On 17/04/2019 08:56, Chris Wilson wrote:
> > Currently there is an underlying assumption that i915_request_unsubmit()
> > is synchronous wrt the GPU -- that is the request is no longer in flight
> > as we remove it. In the near future that may change, and this may upset
> > our signaling as we can process an interrupt for that request while it
> > is no longer in flight.
> 
> Is this the preempt-to-busy future?

Async unsubmit is preempt-to-busy, but one shouldn't rule out the guc
either :)

> 
> > 
> > CPU0                                  CPU1
> > intel_engine_breadcrumbs_irq
> > (queue request completion)
> >                                       i915_request_cancel_signaling
> > ...                                   ...
> >                                       i915_request_enable_signaling
> > dma_fence_signal
> 
> In this case completed request is unsubmitted?

The request completed after it was unsubmitted, yes.

> There will be some inherent problem preventing avoiding preempting 
> completed requests? Window between MI_ARB_ENABLE in emit_fini_breadcrumb 
> and context complete?

For preempt-to-busy, we stick a semaphore into the fini_breadcrumb to
prevent context completion while we perform the preemption.

Please note the second aspect wrt to dereferencing the intel_context
after we skip the cancel_breadcrumbs during retire is why I think this
fixes a present bug.
-Chris