linux-kernel - Re: [PATCH v2 1/6] Btrfs: lock superblock before remounting for rw subvol

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Message-ID: <20150511160738.GP23255@twin.jikos.cz>
Date:	Mon, 11 May 2015 18:07:38 +0200
From:	David Sterba <dsterba@...e.cz>
To:	Omar Sandoval <osandov@...ndov.com>
Cc:	Chris Mason <clm@...com>, Josef Bacik <jbacik@...com>,
	David Sterba <dsterba@...e.cz>, linux-btrfs@...r.kernel.org,
	linux-kernel@...r.kernel.org, Qu Wenruo <quwenruo@...fujitsu.com>
Subject: Re: [PATCH v2 1/6] Btrfs: lock superblock before remounting for rw
 subvol

On Thu, Apr 09, 2015 at 02:34:51PM -0700, Omar Sandoval wrote:
> 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")
> Signed-off-by: Omar Sandoval <osandov@...ndov.com>

Reviewed-by: David Sterba <dsterba@...e.cz>
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