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

[Christoph Hellwig <hch@lst.de>]

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?