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

Dave Chinner
2012-07-19T01:58:16Z

On Wed, Jul 18, 2012 at 12:33:58PM -0500, tinguely@sgi.com 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.

--
Dave Chinner
david@fromorbit.com