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Re: [PATCH] Fix use-after-free with log and quotas

To: Lachlan McIlroy <lachlan@xxxxxxx>, xfs-dev <xfs-dev@xxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
Subject: Re: [PATCH] Fix use-after-free with log and quotas
From: Lachlan McIlroy <lachlan@xxxxxxx>
Date: Tue, 16 Sep 2008 16:36:41 +1000
In-reply-to: <20080916040825.GO5811@disturbed>
References: <48CA2B23.4020405@xxxxxxx> <20080913040219.GA5811@disturbed> <48CDCB04.1040402@xxxxxxx> <20080916040825.GO5811@disturbed>
Reply-to: lachlan@xxxxxxx
User-agent: Thunderbird 2.0.0.16 (X11/20080707)
Dave Chinner wrote:
On Mon, Sep 15, 2008 at 12:40:04PM +1000, Lachlan McIlroy wrote:
Dave Chinner wrote:
On Fri, Sep 12, 2008 at 06:41:07PM +1000, Lachlan McIlroy wrote:
Destroying the quota stuff on unmount can access the log - ie XFS_QM_DONE()
ends up in xfs_dqunlock() which calls xfs_trans_unlocked_item() and then
xfs_log_move_tail().  By this time the log has already been destroyed.
Just move the cleanup of the quota code earlier in xfs_unmountfs() before
the call to xfs_log_unmount().  Moving XFS_QM_DONE() up near
XFS_QM_DQPURGEALL() seems like a good spot.
FWIW, has this been actually seen in the real world?
Yes.  And easy to reproduce too.

Care to provide details about the test case, then? I can't help if
you keep me in the dark....
XFSQA test 083 hits this almost every run when quotas are enabled.


torn down the AIL and there should be no log items in the system
that are in the AIL....
That should be the case but clearly not happening.  Pete is investigating
an issue right now where a dquot is not getting removed from the AIL when
it should.  Until we've got to the bottom of that problem I'd prefer to at
least avoid this use after free issue.

No point in putting a bandaid in if you're already in the process of
trying to find the real cause....
It's not a band-aid - it's a perfectly valid change to make.  We don't
know if this other problem is related nor do we know if it will fix this
use-after-free either.  There's no reason to let other people hit this
panic if we can avoid it.

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