Commit 75764b41 authored by Al Viro's avatar Al Viro Committed by David Sterba
Browse files

btrfs: open code fc_mount() to avoid releasing s_umount rw_sempahore 



[CURRENT BEHAVIOR]
Currently inside btrfs_get_tree_subvol(), we call fc_mount() to grab a
tree, then re-lock s_umount inside btrfs_reconfigure_for_mount() to
avoid race with remount.

However fc_mount() itself is just doing two things:

1. Call vfs_get_tree()
2. Release s_umount then call vfs_create_mount()

[ENHANCEMENT]
Instead of calling fc_mount(), we can open-code it with vfs_get_tree()
first.
This provides a benefit that, since we have the full control of
s_umount, we do not need to re-lock that rw_sempahore when calling
btrfs_reconfigure_for_mount(), meaning less race between RO/RW remount.

Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
Reviewed-by: default avatarQu Wenruo <wqu@suse.com>
[ Rework the subject and commit message, refactor the error handling ]
Signed-off-by: default avatarQu Wenruo <wqu@suse.com>
Tested-by: default avatarQu Wenruo <wqu@suse.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent 4013cde5

fs/btrfs/super.c

+17 −17
Original line number Diff line number Diff line
@@ -1996,17 +1996,13 @@ static int btrfs_get_tree_super(struct fs_context *fc) 
 * btrfs or not, setting the whole super block RO.  To make per-subvolume mounting

 * work with different options work we need to keep backward compatibility.

 */

static int btrfs_reconfigure_for_mount(struct fs_context *fc, struct vfsmount *mnt)

static int btrfs_reconfigure_for_mount(struct fs_context *fc)

{

	int ret = 0;



	if (fc->sb_flags & SB_RDONLY)

		return ret;



	down_write(&mnt->mnt_sb->s_umount);

	if (!(fc->sb_flags & SB_RDONLY) && (mnt->mnt_sb->s_flags & SB_RDONLY))

	if (!(fc->sb_flags & SB_RDONLY) && (fc->root->d_sb->s_flags & SB_RDONLY))

		ret = btrfs_reconfigure(fc);

	up_write(&mnt->mnt_sb->s_umount);



	return ret;

}
@@ -2059,17 +2055,18 @@ static int btrfs_get_tree_subvol(struct fs_context *fc) 
	security_free_mnt_opts(&fc->security);

	fc->security = NULL;



	mnt = fc_mount(dup_fc);

	if (IS_ERR(mnt)) {

	ret = vfs_get_tree(dup_fc);

	if (ret)

		goto error;



	ret = btrfs_reconfigure_for_mount(dup_fc);

	up_write(&dup_fc->root->d_sb->s_umount);

	if (ret)

		goto error;

	mnt = vfs_create_mount(dup_fc);

	put_fs_context(dup_fc);

	if (IS_ERR(mnt))

		return PTR_ERR(mnt);

	}

	ret = btrfs_reconfigure_for_mount(dup_fc, mnt);

	put_fs_context(dup_fc);

	if (ret) {

		mntput(mnt);

		return ret;

	}



	/*

	 * This free's ->subvol_name, because if it isn't set we have to
@@ -2083,6 +2080,9 @@ static int btrfs_get_tree_subvol(struct fs_context *fc) 


	fc->root = dentry;

	return 0;

error:

	put_fs_context(dup_fc);

	return ret;

}



static int btrfs_get_tree(struct fs_context *fc)