Commit 773cd04e authored May 18, 2015 by Omar Sandoval Committed by Chris Mason Jun 03, 2015

Btrfs: lock superblock before remounting for rw subvol



Since commit 0723a047 ("btrfs: allow mounting btrfs subvolumes with
different ro/rw options"), when mounting a subvolume read/write when
another subvolume has previously been mounted read-only, we first do a
remount. However, this should be done with the superblock locked, as per
sync_filesystem():

	/*
	 * We need to be protected against the filesystem going from
	 * r/o to r/w or vice versa.
	 */
	WARN_ON(!rwsem_is_locked(&sb->s_umount));

This WARN_ON can easily be hit with:

mkfs.btrfs -f /dev/vdb
mount /dev/vdb /mnt
btrfs subvol create /mnt/vol1
btrfs subvol create /mnt/vol2
umount /mnt
mount -oro,subvol=/vol1 /dev/vdb /mnt
mount -orw,subvol=/vol2 /dev/vdb /mnt2

Fixes: 0723a047 ("btrfs: allow mounting btrfs subvolumes with different ro/rw options")
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Omar Sandoval <osandov@osandov.com>
Signed-off-by: Chris Mason <clm@fb.com>

parent 0f31871f

fs/btrfs/super.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -1209,7 +1209,9 @@ static struct dentry mount_subvol(const char subvol_name, int flags,
		return ERR_CAST(mnt);
		}

		down_write(&mnt->mnt_sb->s_umount);
		r = btrfs_remount(mnt->mnt_sb, &flags, NULL);
		up_write(&mnt->mnt_sb->s_umount);
		if (r < 0) {
		/* FIXME: release vfsmount mnt ??*/
		kfree(newargs);

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