[PATCH] xfs: copy li_lsn before dropping AIL lock

[Christoph Hellwig]

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.

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.

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