btrfs: open code rcu_string_free() and remove it

The helper is trivial and we can simply use kfree_rcu() if needed. In
our case it's just one place where we rename a device in
device_list_add() and the old name can still be used until the end of
the RCU grace period. The other case is freeing a device and there
nothing should reach the device, so we can use plain kfree().

Reviewed-by: Daniel Vacek <neelx@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: kdave

fs/btrfs/rcu-string.h

@@ -32,12 +32,6 @@ static inline struct rcu_string *rcu_string_strdup(const char *src, gfp_t mask)
 	return ret;
 }
 
-static inline void rcu_string_free(struct rcu_string *str)
-{
-	if (str)
-		kfree_rcu(str, rcu);
-}
-
 #define printk_in_rcu(fmt, ...) do {	\
 	rcu_read_lock();		\
 	printk(fmt, __VA_ARGS__);	\

fs/btrfs/volumes.c

@@ -403,7 +403,8 @@ static struct btrfs_fs_devices *alloc_fs_devices(const u8 *fsid)
 static void btrfs_free_device(struct btrfs_device *device)
 {
 	WARN_ON(!list_empty(&device->post_commit_list));
-	rcu_string_free(device->name);
+	/* No need to call kfree_rcu(), nothing is reading the device name. */
+	kfree(device->name);
 	btrfs_extent_io_tree_release(&device->alloc_state);
 	btrfs_destroy_dev_zone_info(device);
 	kfree(device);
@@ -962,7 +963,7 @@ static noinline struct btrfs_device *device_list_add(const char *path,
 			mutex_unlock(&fs_devices->device_list_mutex);
 			return ERR_PTR(-ENOMEM);
 		}
-		rcu_string_free(device->name);
+		kfree_rcu(device->name, rcu);
 		rcu_assign_pointer(device->name, name);
 		if (test_bit(BTRFS_DEV_STATE_MISSING, &device->dev_state)) {
 			fs_devices->missing_devices--;