[PATCH v3 2/2] drm/i915: Never return 0 if not all requests retired

[Tvrtko Ursulin]

On 21/11/2022 14:56, Janusz Krzysztofik wrote:
> Users of intel_gt_retire_requests_timeout() expect 0 return value on
> success.  However, we have no protection from passing back 0 potentially
> returned by a call to dma_fence_wait_timeout() when it succedes right
> after its timeout has expired.

Is this talking about a potential weakness, or ambiguous kerneldoc, of 
dma_fence_wait_timeout, dma_fence_default_wait and 
i915_request_wait_timeout? They appear to say 0 return means timeout, 
implying unsignaled fence. In other words signaled must return positive 
remaining timeout. Implementations seems to allow a race which indeed 
appears that return 0 and signaled fence is possible.

If dma_fence_wait can indeed return 0 even when a request is signaled, 
then how is timeout ?: -ETIME below correct? It isn't a chance for false 
negative in its' callers?

Regards,

Tvrtko

> Replace 0 with -ETIME before potentially using the timeout value as return
> code, so -ETIME is returned if there are still some requests not retired
> after timeout, 0 otherwise.
> 
> v3: Use conditional expression, more compact but also better reflecting
>      intention standing behind the change.
> 
> v2: Move the added lines down so flush_submission() is not affected.
> 
> Fixes: f33a8a51602c ("drm/i915: Merge wait_for_timelines with retire_request")
> Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik at linux.intel.com>
> Reviewed-by: Andrzej Hajda <andrzej.hajda at intel.com>
> Cc: stable at vger.kernel.org # v5.5+
> ---
>   drivers/gpu/drm/i915/gt/intel_gt_requests.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_gt_requests.c b/drivers/gpu/drm/i915/gt/intel_gt_requests.c
> index edb881d756309..1dfd01668c79c 100644
> --- a/drivers/gpu/drm/i915/gt/intel_gt_requests.c
> +++ b/drivers/gpu/drm/i915/gt/intel_gt_requests.c
> @@ -199,7 +199,7 @@ out_active:	spin_lock(&timelines->lock);
>   	if (remaining_timeout)
>   		*remaining_timeout = timeout;
>   
> -	return active_count ? timeout : 0;
> +	return active_count ? timeout ?: -ETIME : 0;
>   }
>   
>   static void retire_work_handler(struct work_struct *work)