drm/i915: Protect i915_active iterators from the shrinker

If we allocate while iterating the rbtree of active nodes, we may hit
the shrinker and so retire the i915_active, reaping the rbtree. Modifying
the rbtree as we iterate is not good behaviour, so acquire the
i915_active first to keep the tree intact whenever we allocate.

Fixes: a42375a ("drm/i915: Release the active tracker tree upon idling")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190208134704.23039-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/i915_active.c

@@ -163,17 +163,25 @@ int i915_active_ref(struct i915_active *ref,
 		    struct i915_request *rq)
 {
 	struct i915_active_request *active;
+	int err = 0;
+
+	/* Prevent reaping in case we malloc/wait while building the tree */
+	i915_active_acquire(ref);
 
 	active = active_instance(ref, timeline);
-	if (IS_ERR(active))
-		return PTR_ERR(active);
+	if (IS_ERR(active)) {
+		err = PTR_ERR(active);
+		goto out;
+	}
 
 	if (!i915_active_request_isset(active))
 		ref->count++;
 	__i915_active_request_set(active, rq);
 
 	GEM_BUG_ON(!ref->count);
-	return 0;
+out:
+	i915_active_release(ref);
+	return err;
 }
 
 bool i915_active_acquire(struct i915_active *ref)
@@ -223,19 +231,25 @@ int i915_request_await_active_request(struct i915_request *rq,
 int i915_request_await_active(struct i915_request *rq, struct i915_active *ref)
 {
 	struct active_node *it, *n;
-	int ret;
+	int err = 0;
 
-	ret = i915_request_await_active_request(rq, &ref->last);
-	if (ret)
-		return ret;
+	/* await allocates and so we need to avoid hitting the shrinker */
+	if (i915_active_acquire(ref))
+		goto out; /* was idle */
+
+	err = i915_request_await_active_request(rq, &ref->last);
+	if (err)
+		goto out;
 
 	rbtree_postorder_for_each_entry_safe(it, n, &ref->tree, node) {
-		ret = i915_request_await_active_request(rq, &it->base);
-		if (ret)
-			return ret;
+		err = i915_request_await_active_request(rq, &it->base);
+		if (err)
+			goto out;
 	}
 
-	return 0;
+out:
+	i915_active_release(ref);
+	return err;
 }
 
 #if IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM)