Commit 2936a6ac authored by Qu Wenruo's avatar Qu Wenruo Committed by David Sterba
Browse files

btrfs: add assertions to make super block creation more clear 



When calling sget_fc(), there are 3 different situations:

a) Critical error
   No super block created.

b) A new super block is created
   The fc->s_fs_info is transferred to the super block, and fc->s_fs_info
   is reset to NULL.

   In this case sb->s_root should still be NULL, and needs to be properly
   initialized later by btrfs_fill_super().

c) An existing super block is returned
   The fc->s_fs_info is untouched, and anything related to that fs_info
   should be properly cleaned up.

This is not obvious even with the extra comments at sget_fc().

Enhance the situation by:

- Add comments for case b) and c)
  Especially for case c), the fs_info and fs_devices cleanup happens at
  different timing, thus needs extra explanation.

- Move the comments closer to case b) and case c)

Signed-off-by: default avatarQu Wenruo <wqu@suse.com>
Reviewed-by: default avatarDavid Sterba <dsterba@suse.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent 35ea448b

fs/btrfs/super.c

+20 −9
Original line number Diff line number Diff line
@@ -1876,15 +1876,6 @@ static int btrfs_get_tree_super(struct fs_context *fc) 


	bdev = fs_devices->latest_dev->bdev;



	/*

	 * From now on the error handling is not straightforward.

	 *

	 * If successful, this will transfer the fs_info into the super block,

	 * and fc->s_fs_info will be NULL.  However if there's an existing

	 * super, we'll still have fc->s_fs_info populated.  If we error

	 * completely out it'll be cleaned up when we drop the fs_context,

	 * otherwise it's tied to the lifetime of the super_block.

	 */

	sb = sget_fc(fc, btrfs_fc_test_super, set_anon_super_fc);

	if (IS_ERR(sb)) {

		ret = PTR_ERR(sb);
@@ -1894,6 +1885,19 @@ static int btrfs_get_tree_super(struct fs_context *fc) 
	set_device_specific_options(fs_info);



	if (sb->s_root) {

		/*

		 * Not the first mount of the fs thus got an existing super block.

		 * Will reuse the returned super block, fs_info and fs_devices.

		 */

		ASSERT(fc->s_fs_info == fs_info);



		/*

		 * fc->s_fs_info is not touched and will be later freed by

		 * put_fs_context() through btrfs_free_fs_context().

		 *

		 * But we have opened fs_devices at the beginning of the

		 * function, thus still need to close them manually.

		 */

		btrfs_close_devices(fs_devices);

		/*

		 * At this stage we may have RO flag mismatch between
@@ -1902,6 +1906,13 @@ static int btrfs_get_tree_super(struct fs_context *fc) 
		 * needed.

		 */

	} else {

		/*

		 * The first mount of the fs thus a new superblock, fc->s_fs_info

		 * must be NULL, and the ownership of our fs_info and fs_devices is

		 * transferred to the super block.

		 */

		ASSERT(fc->s_fs_info == NULL);



		snprintf(sb->s_id, sizeof(sb->s_id), "%pg", bdev);

		shrinker_debugfs_rename(sb->s_shrink, "sb-btrfs:%s", sb->s_id);

		btrfs_sb(sb)->bdev_holder = &btrfs_fs_type;