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Date: Fri, 17 Oct 2025 05:50:43 +0200
From: Christoph Hellwig <hch@lst.de>
To: "Darrick J. Wong" <djwong@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>, Carlos Maiolino <cem@kernel.org>,
	linux-xfs@vger.kernel.org
Subject: Re: [PATCH 17/17] xfs: reduce ilock roundtrips in
 xfs_qm_vop_dqalloc
Message-ID: <20251017035043.GA29428@lst.de>
References: <20251013024851.4110053-1-hch@lst.de> <20251013024851.4110053-18-hch@lst.de> <20251015212707.GM2591640@frogsfrogsfrogs> <20251016042348.GC29822@lst.de> <20251016155941.GA3356773@frogsfrogsfrogs>
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On Thu, Oct 16, 2025 at 08:59:41AM -0700, Darrick J. Wong wrote:
> > > ...and I guess we no longer detach dquots from live inodes now, so we
> > > really only need ILOCK_EXCL to prevent multiple threads from trying to
> > > allocate and attach a new xfs_dquot object to the same inode, right?
> > 
> > Yes.
> 
> I wonder then, if there /were/ two threads racing to dqattach the same
> inode, won't the radix_tree_insert return EEXIST, preventing us
> from exposing two dquot for the same id because xfs_qm_dqget will just
> loop again?

I think so.

> [Though looking at that xfs_qm_dqget -> xfs_qm_dqget_cache_insert ->
> radix_tree_insert sequence, it looks like we can also restart
> indefinitely on other errors like ENOMEM.]

Yes.  I have patches fixing that as part of moving to xarrays instead
of radix trees.  But I suspect a resizable hash table might actually
be the better fit, so I didn't look into submitting that quite yet.