From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 02/35] Btrfs multi-device code
Date: Tue, 13 Jan 2009 09:52:55 -0800
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	<1231864576.29164.140.camel@think.oraclecorp.com>
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Cc: torvalds@linux-foundation.org, linux-fsdevel@vger.kernel.org,
	Andy Whitcroft <apw@shadowen.org>
To: Chris Mason <chris.mason@oracle.com>
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On Tue, 13 Jan 2009 11:36:16 -0500 Chris Mason <chris.mason@oracle.com> wrote:

> > > +	bdev = open_bdev_exclusive(device_path, 0, root->fs_info->bdev_holder);
> > > +	if (!bdev)
> > > +		return -EIO;
> > > +
> > > +	if (root->fs_info->fs_devices->seeding) {
> > > +		seeding_dev = 1;
> > > +		down_write(&sb->s_umount);
> > > +		mutex_lock(&uuid_mutex);
> > > +	}
> > > +
> > > +	filemap_write_and_wait(bdev->bd_inode->i_mapping);
> > 
> > I'm surprised that this can be safely done under ->s_umount.
> 
> btrfs_init_new_device is only called by the add_dev ioctl.  That doesn't
> happen with s_umount held.  Or do you mean we should be taking s_umount?

I'm surprised that filemap_write_and_wait() can safely be called under
sb->s_umount.  But I guess filemap_write_and_wait() doesn't take many
locks, even though it waits a lot.