FS-Cache: The object-available state can't rely on the cookie to be available

The object-available state in the object processing state machine (as
processed by fscache_object_available()) can't rely on the cookie to be
available because the FSCACHE_COOKIE_CREATING bit may have been cleared by
fscache_obtained_object() prior to the object being put into the
FSCACHE_OBJECT_AVAILABLE state.

Clearing the FSCACHE_COOKIE_CREATING bit on a cookie permits
__fscache_relinquish_cookie() to proceed and detach the cookie from the
object.

To deal with this, we don't dereference object->cookie in
fscache_object_available() if the object has already been detached.

In addition, a couple of assertions are added into fscache_drop_object() to
make sure the object is unbound from the cookie before it gets there.
Signed-off-by: David Howells <dhowells@redhat.com>

fs/fscache/object.c

@@ -158,7 +158,8 @@ static void fscache_object_state_machine(struct fscache_object *object)
 
 		spin_lock(&object->lock);
 		object->state = FSCACHE_OBJECT_DYING;
-		if (test_and_clear_bit(FSCACHE_COOKIE_CREATING,
+		if (object->cookie &&
+		    test_and_clear_bit(FSCACHE_COOKIE_CREATING,
 				       &object->cookie->flags))
 			wake_up_bit(&object->cookie->flags,
 				    FSCACHE_COOKIE_CREATING);
@@ -594,7 +595,8 @@ static void fscache_object_available(struct fscache_object *object)
 
 	spin_lock(&object->lock);
 
-	if (test_and_clear_bit(FSCACHE_COOKIE_CREATING, &object->cookie->flags))
+	if (object->cookie &&
+	    test_and_clear_bit(FSCACHE_COOKIE_CREATING, &object->cookie->flags))
 		wake_up_bit(&object->cookie->flags, FSCACHE_COOKIE_CREATING);
 
 	fscache_done_parent_op(object);
@@ -631,6 +633,9 @@ static void fscache_drop_object(struct fscache_object *object)
 
 	_enter("{OBJ%x,%d}", object->debug_id, object->n_children);
 
+	ASSERTCMP(object->cookie, ==, NULL);
+	ASSERT(hlist_unhashed(&object->cookie_link));
+
 	spin_lock(&cache->object_list_lock);
 	list_del_init(&object->cache_link);
 	spin_unlock(&cache->object_list_lock);