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

@@ -2753,6 +2753,9 @@ 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.
 	 */
+
+	if (i915.enable_execlists) {
+		spin_lock_irq(&ring->execlist_lock);
 		while (!list_empty(&ring->execlist_queue)) {
 			struct drm_i915_gem_request *submit_req;
 
@@ -2766,6 +2769,8 @@ static void i915_gem_reset_ring_cleanup(struct drm_i915_private *dev_priv,
 
 			i915_gem_request_unreference(submit_req);
 		}
+		spin_unlock_irq(&ring->execlist_lock);
+	}
 
 	/*
 	 * We must free the requests after all the corresponding objects have