--- a/src/share/vm/runtime/park.cpp	Tue Apr 01 14:09:03 2014 +0200
+++ b/src/share/vm/runtime/park.cpp	Tue Apr 01 13:57:07 2014 +0200
@@ -59,22 +59,58 @@
 
   // Start by trying to recycle an existing but unassociated
   // ParkEvent from the global free list.
-  // Using a spin lock since we are part of the mutex impl.
-  // 8028280: using concurrent free list without memory management can leak
-  // pretty badly it turns out.
-  Thread::SpinAcquire(&ListLock, "ParkEventFreeListAllocate");
-  {
-    ev = FreeList;
-    if (ev != NULL) {
-      FreeList = ev->FreeNext;
+  for (;;) {
+    ev = FreeList ;
+    if (ev == NULL) break ;
+    // 1: Detach - sequester or privatize the list
+    // Tantamount to ev = Swap (&FreeList, NULL)
+    if (Atomic::cmpxchg_ptr (NULL, &FreeList, ev) != ev) {
+       continue ;
     }
+
+    // We've detached the list.  The list in-hand is now
+    // local to this thread.   This thread can operate on the
+    // list without risk of interference from other threads.
+    // 2: Extract -- pop the 1st element from the list.
+    ParkEvent * List = ev->FreeNext ;
+    if (List == NULL) break ;
+    for (;;) {
+        // 3: Try to reattach the residual list
+        guarantee (List != NULL, "invariant") ;
+        ParkEvent * Arv =  (ParkEvent *) Atomic::cmpxchg_ptr (List, &FreeList, NULL) ;
+        if (Arv == NULL) break ;
+
+        // New nodes arrived.  Try to detach the recent arrivals.
+        if (Atomic::cmpxchg_ptr (NULL, &FreeList, Arv) != Arv) {
+            continue ;
+        }
+        guarantee (Arv != NULL, "invariant") ;
+        // 4: Merge Arv into List
+        ParkEvent * Tail = List ;
+        while (Tail->FreeNext != NULL) Tail = Tail->FreeNext ;
+        Tail->FreeNext = Arv ;
+    }
+    break ;
   }
-  Thread::SpinRelease(&ListLock);
 
   if (ev != NULL) {
     guarantee (ev->AssociatedWith == NULL, "invariant") ;
   } else {
     // Do this the hard way -- materialize a new ParkEvent.
+    // In rare cases an allocating thread might detach a long list --
+    // installing null into FreeList -- and then stall or be obstructed.
+    // A 2nd thread calling Allocate() would see FreeList == null.
+    // The list held privately by the 1st thread is unavailable to the 2nd thread.
+    // In that case the 2nd thread would have to materialize a new ParkEvent,
+    // even though free ParkEvents existed in the system.  In this case we end up
+    // with more ParkEvents in circulation than we need, but the race is
+    // rare and the outcome is benign.  Ideally, the # of extant ParkEvents
+    // is equal to the maximum # of threads that existed at any one time.
+    // Because of the race mentioned above, segments of the freelist
+    // can be transiently inaccessible.  At worst we may end up with the
+    // # of ParkEvents in circulation slightly above the ideal.
+    // Note that if we didn't have the TSM/immortal constraint, then
+    // when reattaching, above, we could trim the list.
     ev = new ParkEvent () ;
     guarantee ((intptr_t(ev) & 0xFF) == 0, "invariant") ;
   }
@@ -88,14 +124,13 @@
   if (ev == NULL) return ;
   guarantee (ev->FreeNext == NULL      , "invariant") ;
   ev->AssociatedWith = NULL ;
-  // Note that if we didn't have the TSM/immortal constraint, then
-  // when reattaching we could trim the list.
-  Thread::SpinAcquire(&ListLock, "ParkEventFreeListRelease");
-  {
-    ev->FreeNext = FreeList;
-    FreeList = ev;
+  for (;;) {
+    // Push ev onto FreeList
+    // The mechanism is "half" lock-free.
+    ParkEvent * List = FreeList ;
+    ev->FreeNext = List ;
+    if (Atomic::cmpxchg_ptr (ev, &FreeList, List) == List) break ;
   }
-  Thread::SpinRelease(&ListLock);
 }
 
 // Override operator new and delete so we can ensure that the
@@ -117,7 +152,7 @@
 
 // 6399321 As a temporary measure we copied & modified the ParkEvent::
 // allocate() and release() code for use by Parkers.  The Parker:: forms
-// will eventually be removed as we consolidate and shift over to ParkEvents
+// will eventually be removed as we consolide and shift over to ParkEvents
 // for both builtin synchronization and JSR166 operations.
 
 volatile int Parker::ListLock = 0 ;
@@ -129,21 +164,56 @@
 
   // Start by trying to recycle an existing but unassociated
   // Parker from the global free list.
-  // 8028280: using concurrent free list without memory management can leak
-  // pretty badly it turns out.
-  Thread::SpinAcquire(&ListLock, "ParkerFreeListAllocate");
-  {
-    p = FreeList;
-    if (p != NULL) {
-      FreeList = p->FreeNext;
+  for (;;) {
+    p = FreeList ;
+    if (p  == NULL) break ;
+    // 1: Detach
+    // Tantamount to p = Swap (&FreeList, NULL)
+    if (Atomic::cmpxchg_ptr (NULL, &FreeList, p) != p) {
+       continue ;
     }
+
+    // We've detached the list.  The list in-hand is now
+    // local to this thread.   This thread can operate on the
+    // list without risk of interference from other threads.
+    // 2: Extract -- pop the 1st element from the list.
+    Parker * List = p->FreeNext ;
+    if (List == NULL) break ;
+    for (;;) {
+        // 3: Try to reattach the residual list
+        guarantee (List != NULL, "invariant") ;
+        Parker * Arv =  (Parker *) Atomic::cmpxchg_ptr (List, &FreeList, NULL) ;
+        if (Arv == NULL) break ;
+
+        // New nodes arrived.  Try to detach the recent arrivals.
+        if (Atomic::cmpxchg_ptr (NULL, &FreeList, Arv) != Arv) {
+            continue ;
+        }
+        guarantee (Arv != NULL, "invariant") ;
+        // 4: Merge Arv into List
+        Parker * Tail = List ;
+        while (Tail->FreeNext != NULL) Tail = Tail->FreeNext ;
+        Tail->FreeNext = Arv ;
+    }
+    break ;
   }
-  Thread::SpinRelease(&ListLock);
 
   if (p != NULL) {
     guarantee (p->AssociatedWith == NULL, "invariant") ;
   } else {
     // Do this the hard way -- materialize a new Parker..
+    // In rare cases an allocating thread might detach
+    // a long list -- installing null into FreeList --and
+    // then stall.  Another thread calling Allocate() would see
+    // FreeList == null and then invoke the ctor.  In this case we
+    // end up with more Parkers in circulation than we need, but
+    // the race is rare and the outcome is benign.
+    // Ideally, the # of extant Parkers is equal to the
+    // maximum # of threads that existed at any one time.
+    // Because of the race mentioned above, segments of the
+    // freelist can be transiently inaccessible.  At worst
+    // we may end up with the # of Parkers in circulation
+    // slightly above the ideal.
     p = new Parker() ;
   }
   p->AssociatedWith = t ;          // Associate p with t
@@ -157,12 +227,11 @@
   guarantee (p->AssociatedWith != NULL, "invariant") ;
   guarantee (p->FreeNext == NULL      , "invariant") ;
   p->AssociatedWith = NULL ;
-
-  Thread::SpinAcquire(&ListLock, "ParkerFreeListRelease");
-  {
-    p->FreeNext = FreeList;
-    FreeList = p;
+  for (;;) {
+    // Push p onto FreeList
+    Parker * List = FreeList ;
+    p->FreeNext = List ;
+    if (Atomic::cmpxchg_ptr (p, &FreeList, List) == List) break ;
   }
-  Thread::SpinRelease(&ListLock);
 }