--- old/src/hotspot/cpu/x86/sharedRuntime_x86_64.cpp	2018-09-26 10:02:17.072885452 -0400
+++ new/src/hotspot/cpu/x86/sharedRuntime_x86_64.cpp	2018-09-26 10:02:15.524795383 -0400
@@ -2464,11 +2464,8 @@
     // Save (object->mark() | 1) into BasicLock's displaced header
     __ movptr(Address(lock_reg, mark_word_offset), swap_reg);
 
-    if (os::is_MP()) {
-      __ lock();
-    }
-
     // src -> dest iff dest == rax else rax <- dest
+    __ lock();
     __ cmpxchgptr(lock_reg, Address(obj_reg, oopDesc::mark_offset_in_bytes()));
     __ jcc(Assembler::equal, lock_done);
 
@@ -2558,19 +2555,17 @@
   //     didn't see any synchronization is progress, and escapes.
   __ movl(Address(r15_thread, JavaThread::thread_state_offset()), _thread_in_native_trans);
 
-  if(os::is_MP()) {
-    if (UseMembar) {
-      // Force this write out before the read below
-      __ membar(Assembler::Membar_mask_bits(
-           Assembler::LoadLoad | Assembler::LoadStore |
-           Assembler::StoreLoad | Assembler::StoreStore));
-    } else {
-      // Write serialization page so VM thread can do a pseudo remote membar.
-      // We use the current thread pointer to calculate a thread specific
-      // offset to write to within the page. This minimizes bus traffic
-      // due to cache line collision.
-      __ serialize_memory(r15_thread, rcx);
-    }
+  if (UseMembar) {
+    // Force this write out before the read below
+    __ membar(Assembler::Membar_mask_bits(
+                Assembler::LoadLoad | Assembler::LoadStore |
+                Assembler::StoreLoad | Assembler::StoreStore));
+  } else {
+    // Write serialization page so VM thread can do a pseudo remote membar.
+    // We use the current thread pointer to calculate a thread specific
+    // offset to write to within the page. This minimizes bus traffic
+    // due to cache line collision.
+    __ serialize_memory(r15_thread, rcx);
   }
 
   Label after_transition;
@@ -2661,9 +2656,7 @@
     __ movptr(old_hdr, Address(rax, 0));
 
     // Atomic swap old header if oop still contains the stack lock
-    if (os::is_MP()) {
-      __ lock();
-    }
+    __ lock();
     __ cmpxchgptr(old_hdr, Address(obj_reg, oopDesc::mark_offset_in_bytes()));
     __ jcc(Assembler::notEqual, slow_path_unlock);