Re: [PATCH 2/2] drm/i915: Check waiter->seqno carefully in case of preemption

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 18/09/2017 17:27, Chris Wilson wrote:
> If preemption occurs at precisely the right moment, we may decide that
> the wait is complete even though the wait's request is no longer
> executing (having been preempted). We handle this situation by double
> checking that request following deciding whether the wait is complete.
> 
> Reported-by: Michał Winiarski <michal.winiarski@intel.com
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Michał Winiarski <michal.winiarski@intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> ---
>   drivers/gpu/drm/i915/i915_irq.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
> index bb69c5b0efc4..7a53d94b7e61 100644
> --- a/drivers/gpu/drm/i915/i915_irq.c
> +++ b/drivers/gpu/drm/i915/i915_irq.c
> @@ -1053,10 +1053,13 @@ static void notify_ring(struct intel_engine_cs *engine)
>   		 */
>   		if (i915_seqno_passed(intel_engine_get_seqno(engine),
>   				      wait->seqno)) {
> +			struct drm_i915_gem_request *waiter = wait->request;
> +
>   			wakeup = true;
>   			if (!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
> -				      &wait->request->fence.flags))
> -				rq = i915_gem_request_get(wait->request);
> +				      &waiter->fence.flags) &&
> +			    intel_wait_check_request(wait, waiter))
> +				rq = i915_gem_request_get(waiter);
>   		}
>   
>   		if (wakeup)
> 

Hm but as the user interrupt is nor serialized to exelists, what 
prevents the preemption to happen after the intel_wait_check_request and 
before dma_fence_signal?

Regards,

Tvrtko
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