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

To: tinguely@xxxxxxx
Subject: Re: [RFC] xfs: wait for the write of the superblock on unmount
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Thu, 19 Jul 2012 11:58:09 +1000
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20120718220003.396849822@xxxxxxxxxxxxxxx>
References: <20120717215957.855744999@xxxxxxxxxxxxxxx> <20120718220003.396849822@xxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
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.

-- 
Dave Chinner
david@xxxxxxxxxxxxx

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