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	id 65FF8227A87; Thu, 16 Oct 2025 06:23:48 +0200 (CEST)
Date: Thu, 16 Oct 2025 06:23:48 +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: <20251016042348.GC29822@lst.de>
References: <20251013024851.4110053-1-hch@lst.de> <20251013024851.4110053-18-hch@lst.de> <20251015212707.GM2591640@frogsfrogsfrogs>
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On Wed, Oct 15, 2025 at 02:27:07PM -0700, Darrick J. Wong wrote:
> On Mon, Oct 13, 2025 at 11:48:18AM +0900, Christoph Hellwig wrote:
> > xfs_qm_vop_dqalloc only needs the (exclusive) ilock for attaching dquots
> > to the inode if not done so yet.  All the other locks don't touch the inode
> > and don't need the ilock - the i_rwsem / iolock protects against changes
> > to the IDs while we are in a method, and the ilock would not help because
> > dropping it for the dqget calls would be racy anyway.
> 
> ...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.

> Changing the i_dquot pointers (aka chown/chproj) is what's coordinated
> under the iolock, right?

Yes.  i_rwsem in the VFS to be specific, but they are the same actual
lock.