drm/i915: Grab execlist spinlock to avoid post-reset concurrency issues.

Grab execlist lock when cleaning up execlist queues after GPU reset to avoid
concurrency problems between the context event interrupt handler and the reset
path immediately following a GPU reset.

* v2 (Chris Wilson):
Do execlist check and use simpler form of spinlock functions.
Signed-off-by: Tomas Elf <tomas.elf@intel.com>
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -2753,18 +2753,23 @@ static void i915_gem_reset_ring_cleanup(struct drm_i915_private *dev_priv,
 	 * are the ones that keep the context and ringbuffer backing objects
 	 * pinned in place.
 	 */
-	while (!list_empty(&ring->execlist_queue)) {
-		struct drm_i915_gem_request *submit_req;
 
-		submit_req = list_first_entry(&ring->execlist_queue,
-				struct drm_i915_gem_request,
-				execlist_link);
-		list_del(&submit_req->execlist_link);
+	if (i915.enable_execlists) {
+		spin_lock_irq(&ring->execlist_lock);
+		while (!list_empty(&ring->execlist_queue)) {
+			struct drm_i915_gem_request *submit_req;
 
-		if (submit_req->ctx != ring->default_context)
-			intel_lr_context_unpin(submit_req);
+			submit_req = list_first_entry(&ring->execlist_queue,
+					struct drm_i915_gem_request,
+					execlist_link);
+			list_del(&submit_req->execlist_link);
 
-		i915_gem_request_unreference(submit_req);
+			if (submit_req->ctx != ring->default_context)
+				intel_lr_context_unpin(submit_req);
+
+			i915_gem_request_unreference(submit_req);
+		}
+		spin_unlock_irq(&ring->execlist_lock);
 	}
 
 	/*