jfs: makes diUnmount/diMount in jfs_mount_rw atomic

jfs_mount_rw can call diUnmount and then diMount. These calls change the
imap pointer. Between these two calls there may be calls of function
jfs_lookup(). The jfs_lookup() function calls jfs_iget(), which, in turn
calls diRead(). The latter references the imap pointer. That may cause
diRead() to refer to a pointer freed in diUnmount().  This commit makes
the calls to diUnmount()/diMount() atomic so that nothing will read the
imap pointer until the whole remount is completed.
Signed-off-by: Oleg Kanatov <okanatov@gmail.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>

fs/jfs/jfs_imap.c

@@ -310,8 +310,8 @@ int diRead(struct inode *ip)
 	iagno = INOTOIAG(ip->i_ino);
 
 	/* read the iag */
-	imap = JFS_IP(ipimap)->i_imap;
 	IREAD_LOCK(ipimap, RDWRLOCK_IMAP);
+	imap = JFS_IP(ipimap)->i_imap;
 	rc = diIAGRead(imap, iagno, &mp);
 	IREAD_UNLOCK(ipimap);
 	if (rc) {

fs/jfs/jfs_mount.c

@@ -234,11 +234,15 @@ int jfs_mount_rw(struct super_block *sb, int remount)
 
 		truncate_inode_pages(sbi->ipimap->i_mapping, 0);
 		truncate_inode_pages(sbi->ipbmap->i_mapping, 0);
+
+		IWRITE_LOCK(sbi->ipimap, RDWRLOCK_IMAP);
 		diUnmount(sbi->ipimap, 1);
 		if ((rc = diMount(sbi->ipimap))) {
+			IWRITE_UNLOCK(sbi->ipimap);
 			jfs_err("jfs_mount_rw: diMount failed!");
 			return rc;
 		}
+		IWRITE_UNLOCK(sbi->ipimap);
 
 		dbUnmount(sbi->ipbmap, 1);
 		if ((rc = dbMount(sbi->ipbmap))) {