Re: [PATCH 17/17] xfs: reduce ilock roundtrips in xfs_qm_vop_dqalloc

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

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?

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

--D

> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/xfs_qm.c | 32 +++-----------------------------
>  1 file changed, 3 insertions(+), 29 deletions(-)
> 
> diff --git a/fs/xfs/xfs_qm.c b/fs/xfs/xfs_qm.c
> index 7fbb89fcdeb9..336de0479022 100644
> --- a/fs/xfs/xfs_qm.c
> +++ b/fs/xfs/xfs_qm.c
> @@ -1861,16 +1861,12 @@ xfs_qm_vop_dqalloc(
>  	struct xfs_dquot	*gq = NULL;
>  	struct xfs_dquot	*pq = NULL;
>  	int			error;
> -	uint			lockflags;
>  
>  	if (!XFS_IS_QUOTA_ON(mp))
>  		return 0;
>  
>  	ASSERT(!xfs_is_metadir_inode(ip));
>  
> -	lockflags = XFS_ILOCK_EXCL;
> -	xfs_ilock(ip, lockflags);
> -
>  	if ((flags & XFS_QMOPT_INHERIT) && XFS_INHERIT_GID(ip))
>  		gid = inode->i_gid;
>  
> @@ -1879,37 +1875,22 @@ xfs_qm_vop_dqalloc(
>  	 * if necessary. The dquot(s) will not be locked.
>  	 */
>  	if (XFS_NOT_DQATTACHED(mp, ip)) {
> +		xfs_ilock(ip, XFS_ILOCK_EXCL);
>  		error = xfs_qm_dqattach_locked(ip, true);
> -		if (error) {
> -			xfs_iunlock(ip, lockflags);
> +		xfs_iunlock(ip, XFS_ILOCK_EXCL);
> +		if (error)
>  			return error;
> -		}
>  	}
>  
>  	if ((flags & XFS_QMOPT_UQUOTA) && XFS_IS_UQUOTA_ON(mp)) {
>  		ASSERT(O_udqpp);
>  		if (!uid_eq(inode->i_uid, uid)) {
> -			/*
> -			 * What we need is the dquot that has this uid, and
> -			 * if we send the inode to dqget, the uid of the inode
> -			 * takes priority over what's sent in the uid argument.
> -			 * We must unlock inode here before calling dqget if
> -			 * we're not sending the inode, because otherwise
> -			 * we'll deadlock by doing trans_reserve while
> -			 * holding ilock.
> -			 */
> -			xfs_iunlock(ip, lockflags);
>  			error = xfs_qm_dqget(mp, from_kuid(user_ns, uid),
>  					XFS_DQTYPE_USER, true, &uq);
>  			if (error) {
>  				ASSERT(error != -ENOENT);
>  				return error;
>  			}
> -			/*
> -			 * Get the ilock in the right order.
> -			 */
> -			lockflags = XFS_ILOCK_SHARED;
> -			xfs_ilock(ip, lockflags);
>  		} else {
>  			/*
>  			 * Take an extra reference, because we'll return
> @@ -1922,15 +1903,12 @@ xfs_qm_vop_dqalloc(
>  	if ((flags & XFS_QMOPT_GQUOTA) && XFS_IS_GQUOTA_ON(mp)) {
>  		ASSERT(O_gdqpp);
>  		if (!gid_eq(inode->i_gid, gid)) {
> -			xfs_iunlock(ip, lockflags);
>  			error = xfs_qm_dqget(mp, from_kgid(user_ns, gid),
>  					XFS_DQTYPE_GROUP, true, &gq);
>  			if (error) {
>  				ASSERT(error != -ENOENT);
>  				goto error_rele;
>  			}
> -			lockflags = XFS_ILOCK_SHARED;
> -			xfs_ilock(ip, lockflags);
>  		} else {
>  			ASSERT(ip->i_gdquot);
>  			gq = xfs_qm_dqhold(ip->i_gdquot);
> @@ -1939,15 +1917,12 @@ xfs_qm_vop_dqalloc(
>  	if ((flags & XFS_QMOPT_PQUOTA) && XFS_IS_PQUOTA_ON(mp)) {
>  		ASSERT(O_pdqpp);
>  		if (ip->i_projid != prid) {
> -			xfs_iunlock(ip, lockflags);
>  			error = xfs_qm_dqget(mp, prid,
>  					XFS_DQTYPE_PROJ, true, &pq);
>  			if (error) {
>  				ASSERT(error != -ENOENT);
>  				goto error_rele;
>  			}
> -			lockflags = XFS_ILOCK_SHARED;
> -			xfs_ilock(ip, lockflags);
>  		} else {
>  			ASSERT(ip->i_pdquot);
>  			pq = xfs_qm_dqhold(ip->i_pdquot);
> @@ -1955,7 +1930,6 @@ xfs_qm_vop_dqalloc(
>  	}
>  	trace_xfs_dquot_dqalloc(ip);
>  
> -	xfs_iunlock(ip, lockflags);
>  	if (O_udqpp)
>  		*O_udqpp = uq;
>  	else
> -- 
> 2.47.3
> 
>