--- a/src/hotspot/share/runtime/os.cpp	Fri Jan 18 17:34:43 2019 -0800
+++ b/src/hotspot/share/runtime/os.cpp	Sat Jan 19 11:19:55 2019 +0100
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2018, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2019, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -217,7 +217,8 @@
 OSReturn os::set_priority(Thread* thread, ThreadPriority p) {
   debug_only(Thread::check_for_dangling_thread_pointer(thread);)
 
-  if (p >= MinPriority && p <= MaxPriority) {
+  if ((p >= MinPriority && p <= MaxPriority) ||
+      (p == CriticalPriority && thread->is_ConcurrentGC_thread())) {
     int priority = java_to_os_priority[p];
     return set_native_priority(thread, priority);
   } else {

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/hotspot/jtreg/gc/cms/TestCriticalPriority.java	Sat Jan 19 11:19:55 2019 +0100
@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2019, Red Hat, Inc. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test TestCriticalPriority
+ * @key gc
+ * @bug 8217378
+ * @requires vm.gc.ConcMarkSweep & !vm.graal.enabled
+ * @summary Test critical priority is accepted
+ * @run main/othervm -XX:+UseConcMarkSweepGC -XX:+UnlockExperimentalVMOptions -XX:+UseCriticalCMSThreadPriority TestCriticalPriority
+ */
+
+public class TestCriticalPriority {
+    public static void main(String args[]) throws Exception {
+        // The failure would be detected before entering main().
+    }
+}