bcache: fix crash with incomplete cache set

Change-Id: I6abde52afe917633480caaf4e2518f42a816d886

drivers/md/bcache/bcache.h

@@ -477,9 +477,13 @@ struct gc_stat {
  * CACHE_SET_STOPPING always gets set first when we're closing down a cache set;
  * we'll continue to run normally for awhile with CACHE_SET_STOPPING set (i.e.
  * flushing dirty data).
+ *
+ * CACHE_SET_RUNNING means all cache devices have been registered and journal
+ * replay is complete.
  */
 #define CACHE_SET_UNREGISTERING		0
 #define	CACHE_SET_STOPPING		1
+#define	CACHE_SET_RUNNING		2
 
 struct cache_set {
 	struct closure		cl;

drivers/md/bcache/super.c

@@ -1284,6 +1284,9 @@ int bch_flash_dev_create(struct cache_set *c, uint64_t size)
 	if (test_bit(CACHE_SET_STOPPING, &c->flags))
 		return -EINTR;
 
+	if (!test_bit(CACHE_SET_RUNNING, &c->flags))
+		return -EPERM;
+
 	u = uuid_find_empty(c);
 	if (!u) {
 		pr_err("Can't create volume, no room for UUID");
@@ -1706,6 +1709,7 @@ static void run_cache_set(struct cache_set *c)
 
 	flash_devs_run(c);
 
+	set_bit(CACHE_SET_RUNNING, &c->flags);
 	return;
 err:
 	closure_sync(&cl);