Re: [PATCH] xfs: copy li_lsn before dropping AIL lock

[Dave Chinner <david@fromorbit.com>]

On Tue, Nov 17, 2009 at 10:13:18AM -0500, Christoph Hellwig wrote:
> On Mon, Nov 16, 2009 at 02:51:48PM -0500, Nathaniel W. Turner wrote:
> > Access to log items on the AIL is generally protected by m_ail_lock;
> > this is particularly needed when we're getting or setting the 64-bit
> > li_lsn on a 32-bit platform.  This patch fixes a couple places where we
> > were accessing the log item after dropping the AIL lock on 32-bit
> > machines.
> >
> > This can result in a partially-zeroed log->l_tail_lsn if
> > xfs_trans_ail_delete is racing with xfs_trans_ail_update, and in at
> > least some cases, this can leave the l_tail_lsn with a zero cycle
> > number, which means xlog_space_left will think the log is full (unless
> > CONFIG_XFS_DEBUG is set, in which case we'll trip an ASSERT), leading to
> > processes stuck forever in xlog_grant_log_space.
> 
> Might this also cause this oops?
> 
> http://www.kerneloops.org/raw.php?rawid=944396&msgid=
> 
> It's been shoving up a few times recently.

I don't think so. That trace is in xfs_close_devices(), which is
called after xfs_unmountfs->xfs_log_unmount->xlog_dealloc, which
tends to imply that mp->m_log is NULL.

i.e. we have IO being flushed when no IO should be pending. Given
that there is a xfs->bdstrat_cb->xfs_bioerror trace in there, it
looks like the buffer could not be written because the system is in
a forced shutdown situation. I have no idea what that buffer might
be or why it wasn't issued and completed during the other flushes
and waits during the unmount process....

> Anyway, the patch looks good to me, but I wonder if we should abstract
> the li_lsn handling a bit more to avoid easily running into this kind of
> problems.

Worth considering, IMO.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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