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

[Mark Tinguely]

On 07/18/12 20:58, Dave Chinner wrote:
> On Wed, Jul 18, 2012 at 12:33:58PM -0500, tinguely at 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.
>

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.