Re: [PATCH] drm/i915/breadcrumbs: Ignore unsubmitted signalers

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

On 06/02/2018 09:46, Chris Wilson wrote:
> When a request is preempted, it is unsubmitted from the HW queue and
> removed from the active list of breadcrumbs. In the process, this
> however triggers the signaler and it may see the clear rbtree with the
> old, and still valid, seqno. This confuses the signaler into action and
> signaling the fence.
> 
> Fixes: d6a2289d9d6b ("drm/i915: Remove the preempted request from the execution queue")
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> Cc: <stable@vger.kernel.org> # v4.12+
> ---
>   drivers/gpu/drm/i915/intel_breadcrumbs.c | 20 ++++----------------
>   1 file changed, 4 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
> index efbc627a2a25..b955f7d7bd0f 100644
> --- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
> +++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
> @@ -588,29 +588,16 @@ void intel_engine_remove_wait(struct intel_engine_cs *engine,
>   	spin_unlock_irq(&b->rb_lock);
>   }
>   
> -static bool signal_valid(const struct drm_i915_gem_request *request)
> -{
> -	return intel_wait_check_request(&request->signaling.wait, request);
> -}
> -
>   static bool signal_complete(const struct drm_i915_gem_request *request)
>   {
>   	if (!request)
>   		return false;
>   
> -	/* If another process served as the bottom-half it may have already
> -	 * signalled that this wait is already completed.
> -	 */
> -	if (intel_wait_complete(&request->signaling.wait))
> -		return signal_valid(request);

Okay so this can return true for unsubmitted requests since rb node will 
be empty and global_seqno == wait.seqno == 0.

I just panic when thinking about races and ordering, since these checks 
used to run unlocked. So even better that they are gone.

> -
> -	/* Carefully check if the request is complete, giving time for the
> +	/*
> +	 * Carefully check if the request is complete, giving time for the
>   	 * seqno to be visible or if the GPU hung.
>   	 */
> -	if (__i915_request_irq_complete(request))
> -		return true;
> -
> -	return false;
> +	return __i915_request_irq_complete(request); >   }
>   
>   static struct drm_i915_gem_request *to_signaler(struct rb_node *rb)
> @@ -712,6 +699,7 @@ static int intel_breadcrumbs_signaler(void *arg)
>   				      &request->fence.flags)) {
>   				local_bh_disable();
>   				dma_fence_signal(&request->fence);
> +				GEM_BUG_ON(!i915_gem_request_completed(request));
>   				local_bh_enable(); /* kick start the tasklets */
>   			}
>   
> 

Looks OK. But I can't say it's straightforward to understand it.

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

Regards,

Tvrtko