Re: [PATCH 7/9] vfs: protect remounting superblock read-only

[Miklos Szeredi]

On Fri, 22 Oct 2010, Al Viro wrote:
> On Tue, Oct 05, 2010 at 12:31:15PM +0200, Miklos Szeredi wrote:
> > From: Miklos Szeredi <mszeredi@xxxxxxx>
> > 
> > Currently remouting superblock read-only is racy in a major way.
> > 
> > With the per mount read-only infrastructure it is now possible to
> > prevent most races, which this patch attempts.
> > 
> > Before starting the remount read-only, set MNT_WRITE_HOLD on all
> > mounts so that writes are held off until the remount either succeeds
> > or fails.
> 
> Umm...  What'll happen if your remount will say mnt_want_write() on
> e.g. internal vfsmount over that sb?

Quickly looking through filesystems I didn't find this sort of usage.

>   Or, more subtle, tries to
> update atime on some opened struct file on that sb.

Hmm, load_nls() will apparently do that.  Drat.

Plan B: remount all vfsmounts read-only before ->remount_fs() but
remember which ones were read-write and restore on failure.  Will
result in ugly transient write failures if remount_fs() fails, but I
don't think anybody would care in practice.

Thanks,
Miklos
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