Re: do_remount_sb(RDONLY) race? (was: XFS oops under 2.6.23.9)

[David Chinner <dgc@sgi.com>]

On Wed, Jan 23, 2008 at 04:24:33PM +1030, Jonathan Woithe wrote:
> > On Wed, Jan 23, 2008 at 03:00:48PM +1030, Jonathan Woithe wrote:
> > > Last night my laptop suffered an oops during closedown.  The full oops
> > > reports can be downloaded from
> > > 
> > >   http://www.atrad.com.au/~jwoithe/xfs_oops/
> > 
> > Assertion failed: atomic_read(&mp->m_active_trans) == 0, file:
> > fs/xfs/xfs_vfsops.c, line 689.
> > 
> > The remount read-only of the root drive supposedly completed
> > while there was still active modification of the filesystem
> > taking place.
.....
> > The read only flag only gets set *after* we've made the filesystem
> > readonly, which means before we are truly read only, we can race
> > with other threads opening files read/write or filesystem
> > modifcations can take place.
> > 
> > The result of that race (if it is really unsafe) will be assert you
> > see. The patch I wrote a couple of months ago to fix the problem
> > is attached below....
> 
> Thanks for the patch.  I will apply it and see what happens.
> 
> Will this be in 2.6.24?

No - because hitting the problem is so rare that I'm not even
sure it's a problem. One of the VFS gurus will need to comment
on whether this really is a problem, and if so the correct fix
is to do_remount_sb() so that it closes the hole for everyone.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group