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

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [RFC] xfs: wait for the write of the superblock on unmount
From: Mark Tinguely <tinguely@xxxxxxx>
Date: Fri, 20 Jul 2012 08:21:59 -0500
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20120719015809.GC23387@dastard>
References: <20120717215957.855744999@xxxxxxxxxxxxxxx> <20120718220003.396849822@xxxxxxxxxxxxxxx> <20120719015809.GC23387@dastard>
User-agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:9.0) Gecko/20120122 Thunderbird/9.0
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.

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