[3.16.y-ckt stable] Patch "Btrfs: lock superblock before remounting for rw subvol" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    Btrfs: lock superblock before remounting for rw subvol

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt15.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 2a590954d70a8a9eeaa0469e620f388ab14b91ae Mon Sep 17 00:00:00 2001
From: Omar Sandoval <osandov at osandov.com>
Date: Mon, 18 May 2015 02:16:26 -0700
Subject: Btrfs: lock superblock before remounting for rw subvol

commit 773cd04ec1911abb33cf9538b65f55b76cad5d92 upstream.

Since commit 0723a0473fb4 ("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: 0723a0473fb4 ("btrfs: allow mounting btrfs subvolumes with different ro/rw options")
Reviewed-by: David Sterba <dsterba at suse.cz>
Signed-off-by: Omar Sandoval <osandov at osandov.com>
Signed-off-by: Chris Mason <clm at fb.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/btrfs/super.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 600035120ca3..863f3d65fe39 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1203,7 +1203,9 @@ 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);