drm/i915: Cope with request list state change during error state capture

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>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

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;