xfs: grab dquots without taking the ilock

Add a new dqget flag that grabs the dquot without taking the ilock.
This will be used by the scrubber (which will have already grabbed
the ilock) to perform basic sanity checking of the quota data.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>

fs/xfs/libxfs/xfs_quota_defs.h

@@ -136,6 +136,8 @@ typedef uint16_t	xfs_qwarncnt_t;
  */
 #define XFS_QMOPT_INHERIT	0x1000000
 
+#define XFS_QMOPT_NOLOCK	0x2000000 /* don't ilock during dqget */
+
 /*
  * flags to xfs_trans_mod_dquot.
  */

fs/xfs/xfs_dquot.c

@@ -472,17 +472,22 @@ xfs_qm_dqtobp(
 	struct xfs_mount	*mp = dqp->q_mount;
 	xfs_dqid_t		id = be32_to_cpu(dqp->q_core.d_id);
 	struct xfs_trans	*tp = (tpp ? *tpp : NULL);
-	uint			lock_mode;
+	uint			lock_mode = 0;
 
 	quotip = xfs_quota_inode(dqp->q_mount, dqp->dq_flags);
 	dqp->q_fileoffset = (xfs_fileoff_t)id / mp->m_quotainfo->qi_dqperchunk;
 
+	ASSERT(!(flags & XFS_QMOPT_NOLOCK) ||
+		xfs_isilocked(quotip, XFS_ILOCK_SHARED) ||
+		xfs_isilocked(quotip, XFS_ILOCK_EXCL));
+	if (!(flags & XFS_QMOPT_NOLOCK))
 		lock_mode = xfs_ilock_data_map_shared(quotip);
 	if (!xfs_this_quota_on(dqp->q_mount, dqp->dq_flags)) {
 		/*
 		 * Return if this type of quotas is turned off while we
 		 * didn't have the quota inode lock.
 		 */
+		if (lock_mode)
 			xfs_iunlock(quotip, lock_mode);
 		return -ESRCH;
 	}
@@ -493,6 +498,7 @@ xfs_qm_dqtobp(
 	error = xfs_bmapi_read(quotip, dqp->q_fileoffset,
 			       XFS_DQUOT_CLUSTER_SIZE_FSB, &map, &nmaps, 0);
 
+	if (lock_mode)
 		xfs_iunlock(quotip, lock_mode);
 	if (error)
 		return error;