xfs: avoid taking the ilock unnessecarily in xfs_qm_dqattach

Check if we actually need to attach a dquot before taking the ilock in
xfs_qm_dqattach.  This avoid superflous lock roundtrips for the common cases
of quota support compiled in but not activated on a filesystem and an
inode that already has the dquots attached.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

fs/xfs/xfs_qm.c

@@ -483,6 +483,23 @@ xfs_qm_dqattach_grouphint(
 	xfs_dqunlock(udq);
 }
 
+static bool
+xfs_qm_need_dqattach(
+	struct xfs_inode	*ip)
+{
+	struct xfs_mount	*mp = ip->i_mount;
+
+	if (!XFS_IS_QUOTA_RUNNING(mp))
+		return false;
+	if (!XFS_IS_QUOTA_ON(mp))
+		return false;
+	if (!XFS_NOT_DQATTACHED(mp, ip))
+		return false;
+	if (ip->i_ino == mp->m_sb.sb_uquotino ||
+	    ip->i_ino == mp->m_sb.sb_gquotino)
+		return false;
+	return true;
+}
 
 /*
  * Given a locked inode, attach dquot(s) to it, taking U/G/P-QUOTAON
@@ -500,11 +517,7 @@ xfs_qm_dqattach_locked(
 	uint		nquotas = 0;
 	int		error = 0;
 
-	if (!XFS_IS_QUOTA_RUNNING(mp) ||
-	    !XFS_IS_QUOTA_ON(mp) ||
-	    !XFS_NOT_DQATTACHED(mp, ip) ||
-	    ip->i_ino == mp->m_sb.sb_uquotino ||
-	    ip->i_ino == mp->m_sb.sb_gquotino)
+	if (!xfs_qm_need_dqattach(ip))
 		return 0;
 
 	ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL));
@@ -575,6 +588,9 @@ xfs_qm_dqattach(
 {
 	int			error;
 
+	if (!xfs_qm_need_dqattach(ip))
+		return 0;
+
 	xfs_ilock(ip, XFS_ILOCK_EXCL);
 	error = xfs_qm_dqattach_locked(ip, flags);
 	xfs_iunlock(ip, XFS_ILOCK_EXCL);