--- a/src/hotspot/share/gc/z/zLock.hpp	Thu Nov 22 09:14:31 2018 +0100
+++ b/src/hotspot/share/gc/z/zLock.hpp	Fri Nov 09 14:08:01 2018 +0100
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, 2017, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 2018, 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
@@ -33,18 +33,35 @@
 
 public:
   ZLock();
+  ~ZLock();
 
   void lock();
   bool try_lock();
   void unlock();
 };
 
+class ZReentrantLock {
+private:
+  ZLock            _lock;
+  Thread* volatile _owner;
+  uint64_t         _count;
+
+public:
+  ZReentrantLock();
+
+  void lock();
+  void unlock();
+
+  bool is_owned() const;
+};
+
+template <typename T>
 class ZLocker : public StackObj {
 private:
-  ZLock* const _lock;
+  T* const _lock;
 
 public:
-  ZLocker(ZLock* lock);
+  ZLocker(T* lock);
   ~ZLocker();
 };
 

--- a/src/hotspot/share/gc/z/zLock.inline.hpp	Thu Nov 22 09:14:31 2018 +0100
+++ b/src/hotspot/share/gc/z/zLock.inline.hpp	Fri Nov 09 14:08:01 2018 +0100
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, 2017, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 2018, 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
@@ -25,11 +25,18 @@
 #define SHARE_GC_Z_ZLOCK_INLINE_HPP
 
 #include "gc/z/zLock.hpp"
+#include "runtime/atomic.hpp"
+#include "runtime/thread.hpp"
+#include "utilities/debug.hpp"
 
 inline ZLock::ZLock() {
   pthread_mutex_init(&_lock, NULL);
 }
 
+inline ZLock::~ZLock() {
+  pthread_mutex_destroy(&_lock);
+}
+
 inline void ZLock::lock() {
   pthread_mutex_lock(&_lock);
 }
@@ -42,12 +49,49 @@
   pthread_mutex_unlock(&_lock);
 }
 
-inline ZLocker::ZLocker(ZLock* lock) :
+inline ZReentrantLock::ZReentrantLock() :
+    _lock(),
+    _owner(NULL),
+    _count(0) {}
+
+inline void ZReentrantLock::lock() {
+  Thread* const thread = Thread::current();
+  Thread* const owner = Atomic::load(&_owner);
+
+  if (owner != thread) {
+    _lock.lock();
+    Atomic::store(thread, &_owner);
+  }
+
+  _count++;
+}
+
+inline void ZReentrantLock::unlock() {
+  assert(is_owned(), "Invalid owner");
+  assert(_count > 0, "Invalid count");
+
+  _count--;
+
+  if (_count == 0) {
+    Atomic::store((Thread*)NULL, &_owner);
+    _lock.unlock();
+  }
+}
+
+inline bool ZReentrantLock::is_owned() const {
+  Thread* const thread = Thread::current();
+  Thread* const owner = Atomic::load(&_owner);
+  return owner == thread;
+}
+
+template <typename T>
+inline ZLocker<T>::ZLocker(T* lock) :
     _lock(lock) {
   _lock->lock();
 }
 
-inline ZLocker::~ZLocker() {
+template <typename T>
+inline ZLocker<T>::~ZLocker() {
   _lock->unlock();
 }
 

--- a/src/hotspot/share/gc/z/zMarkStackAllocator.cpp	Thu Nov 22 09:14:31 2018 +0100
+++ b/src/hotspot/share/gc/z/zMarkStackAllocator.cpp	Fri Nov 09 14:08:01 2018 +0100
@@ -82,7 +82,7 @@
 }
 
 uintptr_t ZMarkStackSpace::expand_and_alloc_space(size_t size) {
-  ZLocker locker(&_expand_lock);
+  ZLocker<ZLock> locker(&_expand_lock);
 
   // Retry allocation before expanding
   uintptr_t addr = alloc_space(size);

--- a/src/hotspot/share/gc/z/zPageAllocator.cpp	Thu Nov 22 09:14:31 2018 +0100
+++ b/src/hotspot/share/gc/z/zPageAllocator.cpp	Fri Nov 09 14:08:01 2018 +0100
@@ -260,7 +260,7 @@
 
   // Free virtual memory
   {
-    ZLocker locker(&_lock);
+    ZLocker<ZLock> locker(&_lock);
     _virtual.free(page->virtual_memory());
   }
 
@@ -268,7 +268,7 @@
 }
 
 void ZPageAllocator::flush_detached_pages(ZList<ZPage>* list) {
-  ZLocker locker(&_lock);
+  ZLocker<ZLock> locker(&_lock);
   list->transfer(&_detached);
 }
 
@@ -376,7 +376,7 @@
       // thread have returned from sem_wait(). To avoid this race we are
       // forcing the waiting thread to acquire/release the lock held by the
       // posting thread. https://sourceware.org/bugzilla/show_bug.cgi?id=12674
-      ZLocker locker(&_lock);
+      ZLocker<ZLock> locker(&_lock);
     }
   }
 
@@ -384,7 +384,7 @@
 }
 
 ZPage* ZPageAllocator::alloc_page_nonblocking(uint8_t type, size_t size, ZAllocationFlags flags) {
-  ZLocker locker(&_lock);
+  ZLocker<ZLock> locker(&_lock);
   return alloc_page_common(type, size, flags);
 }
 
@@ -477,7 +477,7 @@
 }
 
 void ZPageAllocator::free_page(ZPage* page, bool reclaimed) {
-  ZLocker locker(&_lock);
+  ZLocker<ZLock> locker(&_lock);
 
   // Update used statistics
   decrease_used(page->size(), reclaimed);
@@ -495,7 +495,7 @@
 }
 
 void ZPageAllocator::check_out_of_memory() {
-  ZLocker locker(&_lock);
+  ZLocker<ZLock> locker(&_lock);
 
   // Fail allocation requests that were enqueued before the
   // last GC cycle started, otherwise start a new GC cycle.