--- a/src/share/vm/prims/unsafe.cpp	Tue Nov 25 15:59:42 2014 +0100
+++ b/src/share/vm/prims/unsafe.cpp	Fri Dec 19 01:29:51 2014 +0000
@@ -322,10 +322,33 @@
 UNSAFE_END
 
 #ifndef SUPPORTS_NATIVE_CX8
-// Keep old code for platforms which may not have atomic jlong (8 bytes) instructions
 
-// Volatile long versions must use locks if !VM_Version::supports_cx8().
-// support_cx8 is a surrogate for 'supports atomic long memory ops'.
+// VM_Version::supports_cx8() is a surrogate for 'supports atomic long memory ops'.
+//
+// On platforms which do not support atomic compare-and-swap of jlong (8 byte)
+// values we have to use a lock-based scheme to enforce atomicity. This has to be
+// applied to all Unsafe operations that set the value of a jlong field. Even so
+// the compareAndSwapLong operation will not be atomic with respect to direct stores
+// to the field from Java code. It is important therefore that any Java code that
+// utilizes these Unsafe jlong operations does not perform direct stores. To permit
+// direct loads of the field from Java code we must also use Atomic::store within the
+// locked regions. And for good measure, in case there are direct stores, we also
+// employ Atomic::load within those regions. Note that the field in question must be
+// volatile and so must have atomic load/store accesses applied at the Java level.
+//
+// The locking scheme could utilize a range of strategies for controlling the locking
+// granularity: from a lock per-field through to a single global lock. The latter is
+// the simplest and is used for the current implementation. Note that the Java object
+// that contains the field, can not, in general, be used for locking. To do so can lead
+// to deadlocks as we may introduce locking into what appears to the Java code to be a
+// lock-free path.
+//
+// As all the locked-regions are very short and themselves non-blocking we can treat
+// them as leaf routines and elide safepoint checks (ie we don't perform any thread
+// state transitions even when blocking for the lock). Note that if we do choose to
+// add safepoint checks and thread state transitions, we must ensure that we calculate
+// the address of the field _after_ we have acquired the lock, else the object may have
+// been moved by the GC
 
 UNSAFE_ENTRY(jlong, Unsafe_GetLongVolatile(JNIEnv *env, jobject unsafe, jobject obj, jlong offset))
   UnsafeWrapper("Unsafe_GetLongVolatile");
@@ -337,8 +360,8 @@
     else {
       Handle p (THREAD, JNIHandles::resolve(obj));
       jlong* addr = (jlong*)(index_oop_from_field_offset_long(p(), offset));
-      ObjectLocker ol(p, THREAD);
-      jlong value = *addr;
+      MutexLockerEx mu(UnsafeJlong_lock, Mutex::_no_safepoint_check_flag);
+      jlong value = Atomic::load(addr);
       return value;
     }
   }
@@ -353,8 +376,8 @@
     else {
       Handle p (THREAD, JNIHandles::resolve(obj));
       jlong* addr = (jlong*)(index_oop_from_field_offset_long(p(), offset));
-      ObjectLocker ol(p, THREAD);
-      *addr = x;
+      MutexLockerEx mu(UnsafeJlong_lock, Mutex::_no_safepoint_check_flag);
+      Atomic::store(x, addr);
     }
   }
 UNSAFE_END
@@ -463,8 +486,8 @@
     else {
       Handle p (THREAD, JNIHandles::resolve(obj));
       jlong* addr = (jlong*)(index_oop_from_field_offset_long(p(), offset));
-      ObjectLocker ol(p, THREAD);
-      *addr = x;
+      MutexLockerEx mu(UnsafeJlong_lock, Mutex::_no_safepoint_check_flag);
+      Atomic::store(x, addr);
     }
   }
 #endif
@@ -1213,14 +1236,19 @@
   UnsafeWrapper("Unsafe_CompareAndSwapLong");
   Handle p (THREAD, JNIHandles::resolve(obj));
   jlong* addr = (jlong*)(index_oop_from_field_offset_long(p(), offset));
+#ifdef SUPPORTS_NATIVE_CX8
+  return (jlong)(Atomic::cmpxchg(x, addr, e)) == e;
+#else
   if (VM_Version::supports_cx8())
     return (jlong)(Atomic::cmpxchg(x, addr, e)) == e;
   else {
     jboolean success = false;
-    ObjectLocker ol(p, THREAD);
-    if (*addr == e) { *addr = x; success = true; }
+    MutexLockerEx mu(UnsafeJlong_lock, Mutex::_no_safepoint_check_flag);
+    jlong val = Atomic::load(addr);
+    if (val == e) { Atomic::store(x, addr); success = true; }
     return success;
   }
+#endif
 UNSAFE_END
 
 UNSAFE_ENTRY(void, Unsafe_Park(JNIEnv *env, jobject unsafe, jboolean isAbsolute, jlong time))

--- a/src/share/vm/runtime/mutexLocker.cpp	Tue Nov 25 15:59:42 2014 +0100
+++ b/src/share/vm/runtime/mutexLocker.cpp	Fri Dec 19 01:29:51 2014 +0000
@@ -135,6 +135,10 @@
 Mutex*   JfrThreadGroups_lock         = NULL;
 #endif
 
+#ifndef SUPPORTS_NATIVE_CX8
+Mutex*   UnsafeJlong_lock             = NULL;
+#endif
+
 #define MAX_NUM_MUTEX 128
 static Monitor * _mutex_array[MAX_NUM_MUTEX];
 static int _num_mutex;
@@ -286,6 +290,9 @@
   def(JfrStacktrace_lock           , Mutex,   special,     true );
 #endif
 
+#ifndef SUPPORTS_NATIVE_CX8
+  def(UnsafeJlong_lock             , Mutex,   special,     false);
+#endif
 }
 
 GCMutexLocker::GCMutexLocker(Monitor * mutex) {

--- a/src/share/vm/runtime/mutexLocker.hpp	Tue Nov 25 15:59:42 2014 +0100
+++ b/src/share/vm/runtime/mutexLocker.hpp	Fri Dec 19 01:29:51 2014 +0000
@@ -151,6 +151,10 @@
 extern Mutex*   JfrThreadGroups_lock;            // protects JFR access to Thread Groups
 #endif
 
+#ifndef SUPPORTS_NATIVE_CX8
+extern Mutex*   UnsafeJlong_lock;                // provides Unsafe atomic updates to jlongs on platforms that don't support cx8
+#endif
+
 // A MutexLocker provides mutual exclusion with respect to a given mutex
 // for the scope which contains the locker.  The lock is an OS lock, not
 // an object lock, and the two do not interoperate.  Do not use Mutex-based