From: Liu Bo <liubo2009@cn.fujitsu.com>
Date: Fri, 29 Jun 2012 09:58:49 +0000 (-0600)
Subject: Btrfs: do not set subvolume flags in readonly mode
X-Git-Tag: v3.6-rc1~81^2~26
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=b9ca0664dc806ba70587f6f3202b60dc736cd6e5;p=pandora-kernel.git

Btrfs: do not set subvolume flags in readonly mode

$ mkfs.btrfs /dev/sdb7
$ btrfstune -S1 /dev/sdb7
$ mount /dev/sdb7 /mnt/btrfs
mount: block device /dev/sdb7 is write-protected, mounting read-only
$ btrfs dev add /dev/sdb8 /mnt/btrfs/

Now we get a btrfs in which mnt flags has readonly but sb flags does
not.  So for those ioctls that only check sb flags with MS_RDONLY, it
is going to be a problem.
Setting subvolume flags is such an ioctl, we should use mnt_want_write_file()
to check RO flags.

Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com>
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