Re: [RFC] xfs: wait for the write of the superblock on unmount

[Mark Tinguely <tinguely@xxxxxxx>]

On 07/20/12 08:21, Mark Tinguely wrote:
> On 07/18/12 20:58, Dave Chinner wrote:
> > On Wed, Jul 18, 2012 at 12:33:58PM -0500, tinguely@xxxxxxx wrote:
> > > Sorry, I have been distracted away from this regression. This was
> > > previously
> > > titled "xfs: synchronously write the superblock on unmount".
> > >
> > > xfs_wait_buftarg() does not wait for the completion of the write of the
> > > uncached superblock. This write can race with the shutdown of the log
> > > and
> > > causes a panic if the write does not win the race.
> > >
> > > The log write of the superblock is important for possible recovery,
> > > but a
> > > second syncronous write of the same superblock seems redundant. Would
> > > just
> > > waiting for the iodone() of the log write before tearing down the log be
> > > enough?
> > Yes. i.e. something like:
> >
> > /*
> > * The superblock buffer is uncached, so xfs_wait_buftarg()
> > * will not wait for it. Hence we need to explicitly wait
> > * for IO completion on the superblock to occur here.
> > */
> > error = xfs_buf_iowait(mp->m_sb_bp);
> > if (error)
> > AAAAIEEEE!
> >
> > This fix is also needed in xfs_quiesce_attr() for the freeze and
> > ro,remount cases as well.
> >
> > Cheers,
> >
> > Dave.
> Thanks for the feedback.
>
> I chose the ugly lock to block on the superblock buffer because it has
> XBF_ASYNC set at that point and won't go through the complete() patch. I
> will look at turning the XBF_ASYNC flag off and wait on a xfs_buf_iowait().
>
> --Mark.
Looks like the xfsaild_push() sets the XBF_ASYNC flag on the superblock 
buffer. Waiting on the xfs_buf_iowait() won't work but have to use the 
less elegant lock.
--Mark.