[XFS] Prevent lockdep false positives when locking two inodes.

If we call xfs_lock_two_inodes() to grab both the iolock and the ilock,
then drop the ilocks on both inodes, then grab them again (as
xfs_swap_extents() does) then lockdep will report a locking order problem.
This is a false positive.

To avoid this, disallow xfs_lock_two_inodes() fom locking both inode locks
at once - force calers to make two separate calls. This means that nested
dropping and regaining of the ilocks will retain the same lockdep subclass
and so lockdep will not see anything wrong with this code.

SGI-PV: 986238

SGI-Modid: xfs-linux-melb:xfs-kern:31999a

Signed-off-by: David Chinner <[e-mail]>
Signed-off-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Peter Leckie <[e-mail]>
Signed-off-by: Lachlan McIlroy <[e-mail]>

fs/xfs/xfs_dfrag.c

···
 
 	sbp = &sxp->sx_stat;
 
-	xfs_lock_two_inodes(ip, tip, lock_flags);
+	/*
+	 * we have to do two separate lock calls here to keep lockdep
+	 * happy. If we try to get all the locks in one call, lock will
+	 * report false positives when we drop the ILOCK and regain them
+	 * below.
+	 */
+	xfs_lock_two_inodes(ip, tip, XFS_IOLOCK_EXCL);
+	xfs_lock_two_inodes(ip, tip, XFS_ILOCK_EXCL);
 	locked = 1;
 
 	/* Verify that both files have the same format */

fs/xfs/xfs_vnodeops.c

···
 #endif
 }
 
+/*
+ * xfs_lock_two_inodes() can only be used to lock one type of lock
+ * at a time - the iolock or the ilock, but not both at once. If
+ * we lock both at once, lockdep will report false positives saying
+ * we have violated locking orders.
+ */
 void
 xfs_lock_two_inodes(
 	xfs_inode_t		*ip0,
···
 	int			attempts = 0;
 	xfs_log_item_t		*lp;
 
+	if (lock_mode & (XFS_IOLOCK_SHARED|XFS_IOLOCK_EXCL))
+		ASSERT((lock_mode & (XFS_ILOCK_SHARED|XFS_ILOCK_EXCL)) == 0);
 	ASSERT(ip0->i_ino != ip1->i_ino);
 
 	if (ip0->i_ino > ip1->i_ino) {