Re: [PATCH] xfs: retry dqpurge when dquot buffer is busy

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Jun 29, 2026 at 02:43:56PM +0200, Christoph Hellwig wrote:
> On Fri, Jun 26, 2026 at 01:52:57PM -0700, Darrick J. Wong wrote:
> > On Fri, Jun 26, 2026 at 05:52:53PM +0800, Yingjie Gao wrote:
> > > xfs_qm_dqpurge() marks a zero-reference dquot dead before trying to flush
> > > a dirty dquot. If the attached buffer is busy, xfs_dquot_use_attached_buf()
> > > returns -EAGAIN.
> > > 
> > > The error path restores q_lockref.count but then jumps to out_funlock,
> > > which continues into the successful purge tail and destroys the dquot.  At
> > > that point the attached buffer has not been detached and the dquot log item
> > > may still be in the AIL.
> > > 
> > > Restore the retry behavior by dropping the locks and returning -EAGAIN
> > > after resurrecting the lockref.
> > > 
> > > Link: https://lore.kernel.org/linux-xfs/20260625175519.GF6078@frogsfrogsfrogs/
> > > Fixes: 0c5e80bd579f ("xfs: use a lockref for the xfs_dquot reference count")
> > > Cc: stable@vger.kernel.org # v6.19+
> > > Signed-off-by: Yingjie Gao <gaoyingjie@uniontech.com>
> > 
> > Yeah, that's more like what we did before 0c5e80bd579f.  I think the
> > lockref resurrection part still looks ok, but maybe hch has an opinion?
> 
> But is it the right thing?  In dqpurge we really want to kill of the
> dquot, so doing a trylock is not very useful, so we really should not
> do a trylock here but just lock the buffer.
> 
> Darrick, do you remember if there's any lock order inversions we need
> to care about here?

I don't remember off the top of my head, other than (afaict) the trylock
might be a remnant of not wanting to block in quotaoff purge while the
QUOTAOFF log item is also pinning the log tail.

--D