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

[Christoph Hellwig]

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?

This doesn't look beautiful, but I suspect there's no good way around
it.  Can you add your explanation from the reply on why xfs_buf_iowait
does not work here to the comment above the lock/unlock pair?

With that:

Reviewed-by: Christoph Hellwig <hch at lst.de>