Re: [PATCH v3 7/8] drm/i915: Cope with request list state change during error state capture

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Oct 19, 2015 at 05:51:57PM +0100, Tomas Elf wrote:
> Since we're not synchronizing the ring request list during error state capture
> the request list state might change between the time the corresponding error
> request list was allocated and dimensioned to the time when the ring request
> list is actually captured into the error state. If this happens then do an
> early exit and be aware that the captured error state might not be fully
> reliable.
> 
> * v2:
> - Chris Wilson: Removed WARN_ON from size check since having the error state
>   request list and the live driver request list diverge like this is a
>   legitimate behaviour.
> 
> - Tomas Elf: Removed update of num_request field since this made no sense. Just
>   exit and move on.
> 
> * v3:
> - Chris Wilson: Removed error message at the point of early exit. The user is
>   not interested in any state changes happening during the error state capture,
>   only in the state that we're trying to capture at the point of the error.
> 
> Signed-off-by: Tomas Elf <tomas.elf@intel.com>

Queued for -next, thanks for the patch.
-Daniel

> ---
>  drivers/gpu/drm/i915/i915_gpu_error.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gpu_error.c b/drivers/gpu/drm/i915/i915_gpu_error.c
> index 2f04e4f..f3dc67b 100644
> --- a/drivers/gpu/drm/i915/i915_gpu_error.c
> +++ b/drivers/gpu/drm/i915/i915_gpu_error.c
> @@ -1071,6 +1071,25 @@ static void i915_gem_record_rings(struct drm_device *dev,
>  		list_for_each_entry(request, &ring->request_list, list) {
>  			struct drm_i915_error_request *erq;
>  
> +			if (count >= error->ring[i].num_requests) {
> +				/*
> +				 * If the ring request list was changed in
> +				 * between the point where the error request
> +				 * list was created and dimensioned and this
> +				 * point then just exit early to avoid crashes.
> +				 *
> +				 * We don't need to communicate that the
> +				 * request list changed state during error
> +				 * state capture and that the error state is
> +				 * slightly incorrect as a consequence since we
> +				 * are typically only interested in the request
> +				 * list state at the point of error state
> +				 * capture, not in any changes happening during
> +				 * the capture.
> +				 */
> +				break;
> +			}
> +
>  			erq = &error->ring[i].requests[count++];
>  			erq->seqno = request->seqno;
>  			erq->jiffies = request->emitted_jiffies;
> -- 
> 1.9.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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