src/hotspot/share/runtime/threadSMR.cpp

rev 48154 : 8191789: migrate more Thread-SMR stuff from thread.[ch]pp -> threadSMR.[ch]pp
Reviewed-by:

@@ -21,52 +21,465 @@
  * questions.
  *
  */
 
 #include "precompiled.hpp"
+#include "logging/logStream.hpp"
 #include "memory/allocation.inline.hpp"
 #include "runtime/thread.inline.hpp"
-#include "runtime/threadSMR.hpp"
+#include "runtime/threadSMR.inline.hpp"
 #include "services/threadService.hpp"
+#include "utilities/globalDefinitions.hpp"
+#include "utilities/resourceHash.hpp"
+
+Monitor*              ThreadsSMRSupport::_smr_delete_lock =
+                          new Monitor(Monitor::special, "smr_delete_lock",
+                                      false /* allow_vm_block */,
+                                      Monitor::_safepoint_check_never);
+// The '_cnt', '_max' and '_times" fields are enabled via
+// -XX:+EnableThreadSMRStatistics:
+
+// # of parallel threads in _smr_delete_lock->wait().
+// Impl note: Hard to imagine > 64K waiting threads so this could be 16-bit,
+// but there is no nice 16-bit _FORMAT support.
+uint                  ThreadsSMRSupport::_smr_delete_lock_wait_cnt = 0;
+
+// Max # of parallel threads in _smr_delete_lock->wait().
+// Impl note: See _smr_delete_lock_wait_cnt note.
+uint                  ThreadsSMRSupport::_smr_delete_lock_wait_max = 0;
+
+// Flag to indicate when an _smr_delete_lock->notify() is needed.
+// Impl note: See _smr_delete_lock_wait_cnt note.
+volatile uint         ThreadsSMRSupport::_smr_delete_notify = 0;
+
+// # of threads deleted over VM lifetime.
+// Impl note: Atomically incremented over VM lifetime so use unsigned for more
+// range. Unsigned 64-bit would be more future proof, but 64-bit atomic inc
+// isn't available everywhere (or is it?).
+volatile uint         ThreadsSMRSupport::_smr_deleted_thread_cnt = 0;
+
+// Max time in millis to delete a thread.
+// Impl note: 16-bit might be too small on an overloaded machine. Use
+// unsigned since this is a time value. Set via Atomic::cmpxchg() in a
+// loop for correctness.
+volatile uint         ThreadsSMRSupport::_smr_deleted_thread_time_max = 0;
+
+// Cumulative time in millis to delete threads.
+// Impl note: Atomically added to over VM lifetime so use unsigned for more
+// range. Unsigned 64-bit would be more future proof, but 64-bit atomic inc
+// isn't available everywhere (or is it?).
+volatile uint         ThreadsSMRSupport::_smr_deleted_thread_times = 0;
+
+ThreadsList* volatile ThreadsSMRSupport::_smr_java_thread_list = new ThreadsList(0);
+
+// # of ThreadsLists allocated over VM lifetime.
+// Impl note: We allocate a new ThreadsList for every thread create and
+// every thread delete so we need a bigger type than the
+// _smr_deleted_thread_cnt field.
+uint64_t              ThreadsSMRSupport::_smr_java_thread_list_alloc_cnt = 1;
+
+// # of ThreadsLists freed over VM lifetime.
+// Impl note: See _smr_java_thread_list_alloc_cnt note.
+uint64_t              ThreadsSMRSupport::_smr_java_thread_list_free_cnt = 0;
+
+// Max size ThreadsList allocated.
+// Impl note: Max # of threads alive at one time should fit in unsigned 32-bit.
+uint                  ThreadsSMRSupport::_smr_java_thread_list_max = 0;
+
+// Max # of nested ThreadsLists for a thread.
+// Impl note: Hard to imagine > 64K nested ThreadsLists so this could be
+// 16-bit, but there is no nice 16-bit _FORMAT support.
+uint                  ThreadsSMRSupport::_smr_nested_thread_list_max = 0;
+
+// # of ThreadsListHandles deleted over VM lifetime.
+// Impl note: Atomically incremented over VM lifetime so use unsigned for
+// more range. There will be fewer ThreadsListHandles than threads so
+// unsigned 32-bit should be fine.
+volatile uint         ThreadsSMRSupport::_smr_tlh_cnt = 0;
+
+// Max time in millis to delete a ThreadsListHandle.
+// Impl note: 16-bit might be too small on an overloaded machine. Use
+// unsigned since this is a time value. Set via Atomic::cmpxchg() in a
+// loop for correctness.
+volatile uint         ThreadsSMRSupport::_smr_tlh_time_max = 0;
+
+// Cumulative time in millis to delete ThreadsListHandles.
+// Impl note: Atomically added to over VM lifetime so use unsigned for more
+// range. Unsigned 64-bit would be more future proof, but 64-bit atomic inc
+// isn't available everywhere (or is it?).
+volatile uint         ThreadsSMRSupport::_smr_tlh_times = 0;
+
+ThreadsList*          ThreadsSMRSupport::_smr_to_delete_list = NULL;
+
+// # of parallel ThreadsLists on the to-delete list.
+// Impl note: Hard to imagine > 64K ThreadsLists needing to be deleted so
+// this could be 16-bit, but there is no nice 16-bit _FORMAT support.
+uint                  ThreadsSMRSupport::_smr_to_delete_list_cnt = 0;
+
+// Max # of parallel ThreadsLists on the to-delete list.
+// Impl note: See _smr_to_delete_list_cnt note.
+uint                  ThreadsSMRSupport::_smr_to_delete_list_max = 0;
+
+
+// 'inline' functions first so the definitions are before first use:
+
+inline void ThreadsSMRSupport::add_smr_deleted_thread_times(uint add_value) {
+  Atomic::add(add_value, &_smr_deleted_thread_times);
+}
+
+inline void ThreadsSMRSupport::inc_smr_deleted_thread_cnt() {
+  Atomic::inc(&_smr_deleted_thread_cnt);
+}
+
+inline void ThreadsSMRSupport::update_smr_deleted_thread_time_max(uint new_value) {
+  while (true) {
+    uint cur_value = _smr_deleted_thread_time_max;
+    if (new_value <= cur_value) {
+      // No need to update max value so we're done.
+      break;
+    }
+    if (Atomic::cmpxchg(new_value, &_smr_deleted_thread_time_max, cur_value) == cur_value) {
+      // Updated max value so we're done. Otherwise try it all again.
+      break;
+    }
+  }
+}
+
+
+// Hash table of pointers found by a scan. Used for collecting hazard
+// pointers (ThreadsList references). Also used for collecting JavaThreads
+// that are indirectly referenced by hazard ptrs. An instance of this
+// class only contains one type of pointer.
+//
+class ThreadScanHashtable : public CHeapObj<mtThread> {
+ private:
+  static bool ptr_equals(void * const& s1, void * const& s2) {
+    return s1 == s2;
+  }
+
+  static unsigned int ptr_hash(void * const& s1) {
+    // 2654435761 = 2^32 * Phi (golden ratio)
+    return (unsigned int)(((uint32_t)(uintptr_t)s1) * 2654435761u);
+  }
+
+  int _table_size;
+  // ResourceHashtable SIZE is specified at compile time so our
+  // dynamic _table_size is unused for now; 1031 is the first prime
+  // after 1024.
+  typedef ResourceHashtable<void *, int, &ThreadScanHashtable::ptr_hash,
+                            &ThreadScanHashtable::ptr_equals, 1031,
+                            ResourceObj::C_HEAP, mtThread> PtrTable;
+  PtrTable * _ptrs;
+
+ public:
+  // ResourceHashtable is passed to various functions and populated in
+  // different places so we allocate it using C_HEAP to make it immune
+  // from any ResourceMarks that happen to be in the code paths.
+  ThreadScanHashtable(int table_size) : _table_size(table_size), _ptrs(new (ResourceObj::C_HEAP, mtThread) PtrTable()) {}
+
+  ~ThreadScanHashtable() { delete _ptrs; }
+
+  bool has_entry(void *pointer) {
+    int *val_ptr = _ptrs->get(pointer);
+    return val_ptr != NULL && *val_ptr == 1;
+  }
+
+  void add_entry(void *pointer) {
+    _ptrs->put(pointer, 1);
+  }
+};
+
+// Closure to gather JavaThreads indirectly referenced by hazard ptrs
+// (ThreadsList references) into a hash table. This closure handles part 2
+// of the dance - adding all the JavaThreads referenced by the hazard
+// pointer (ThreadsList reference) to the hash table.
+//
+class AddThreadHazardPointerThreadClosure : public ThreadClosure {
+ private:
+  ThreadScanHashtable *_table;
+
+ public:
+  AddThreadHazardPointerThreadClosure(ThreadScanHashtable *table) : _table(table) {}
+
+  virtual void do_thread(Thread *thread) {
+    if (!_table->has_entry((void*)thread)) {
+      // The same JavaThread might be on more than one ThreadsList or
+      // more than one thread might be using the same ThreadsList. In
+      // either case, we only need a single entry for a JavaThread.
+      _table->add_entry((void*)thread);
+    }
+  }
+};
+
+// Closure to gather JavaThreads indirectly referenced by hazard ptrs
+// (ThreadsList references) into a hash table. This closure handles part 1
+// of the dance - hazard ptr chain walking and dispatch to another
+// closure.
+//
+class ScanHazardPtrGatherProtectedThreadsClosure : public ThreadClosure {
+ private:
+  ThreadScanHashtable *_table;
+ public:
+  ScanHazardPtrGatherProtectedThreadsClosure(ThreadScanHashtable *table) : _table(table) {}
+
+  virtual void do_thread(Thread *thread) {
+    assert_locked_or_safepoint(Threads_lock);
+
+    if (thread == NULL) return;
+
+    // This code races with ThreadsSMRSupport::acquire_stable_list() which
+    // is lock-free so we have to handle some special situations.
+    //
+    ThreadsList *current_list = NULL;
+    while (true) {
+      current_list = thread->get_threads_hazard_ptr();
+      // No hazard ptr so nothing more to do.
+      if (current_list == NULL) {
+        assert(thread->get_nested_threads_hazard_ptr() == NULL,
+               "cannot have a nested hazard ptr with a NULL regular hazard ptr");
+        return;
+      }
+
+      // If the hazard ptr is verified as stable (since it is not tagged),
+      // then it is safe to use.
+      if (!Thread::is_hazard_ptr_tagged(current_list)) break;
+
+      // The hazard ptr is tagged as not yet verified as being stable
+      // so we are racing with acquire_stable_list(). This exchange
+      // attempts to invalidate the hazard ptr. If we win the race,
+      // then we can ignore this unstable hazard ptr and the other
+      // thread will retry the attempt to publish a stable hazard ptr.
+      // If we lose the race, then we retry our attempt to look at the
+      // hazard ptr.
+      if (thread->cmpxchg_threads_hazard_ptr(NULL, current_list) == current_list) return;
+    }
+
+    // The current JavaThread has a hazard ptr (ThreadsList reference)
+    // which might be _smr_java_thread_list or it might be an older
+    // ThreadsList that has been removed but not freed. In either case,
+    // the hazard ptr is protecting all the JavaThreads on that
+    // ThreadsList.
+    AddThreadHazardPointerThreadClosure add_cl(_table);
+    current_list->threads_do(&add_cl);
+
+    // Any NestedThreadsLists are also protecting JavaThreads so
+    // gather those also; the ThreadsLists may be different.
+    for (NestedThreadsList* node = thread->get_nested_threads_hazard_ptr();
+         node != NULL; node = node->next()) {
+      node->t_list()->threads_do(&add_cl);
+    }
+  }
+};
+
+// Closure to gather hazard ptrs (ThreadsList references) into a hash table.
+//
+class ScanHazardPtrGatherThreadsListClosure : public ThreadClosure {
+ private:
+  ThreadScanHashtable *_table;
+ public:
+  ScanHazardPtrGatherThreadsListClosure(ThreadScanHashtable *table) : _table(table) {}
+
+  virtual void do_thread(Thread* thread) {
+    assert_locked_or_safepoint(Threads_lock);
+
+    if (thread == NULL) return;
+    ThreadsList *threads = thread->get_threads_hazard_ptr();
+    if (threads == NULL) {
+      assert(thread->get_nested_threads_hazard_ptr() == NULL,
+             "cannot have a nested hazard ptr with a NULL regular hazard ptr");
+      return;
+    }
+    // In this closure we always ignore the tag that might mark this
+    // hazard ptr as not yet verified. If we happen to catch an
+    // unverified hazard ptr that is subsequently discarded (not
+    // published), then the only side effect is that we might keep a
+    // to-be-deleted ThreadsList alive a little longer.
+    threads = Thread::untag_hazard_ptr(threads);
+    if (!_table->has_entry((void*)threads)) {
+      _table->add_entry((void*)threads);
+    }
+
+    // Any NestedThreadsLists are also protecting JavaThreads so
+    // gather those also; the ThreadsLists may be different.
+    for (NestedThreadsList* node = thread->get_nested_threads_hazard_ptr();
+         node != NULL; node = node->next()) {
+      threads = node->t_list();
+      if (!_table->has_entry((void*)threads)) {
+        _table->add_entry((void*)threads);
+      }
+    }
+  }
+};
+
+// Closure to print JavaThreads that have a hazard ptr (ThreadsList
+// reference) that contains an indirect reference to a specific JavaThread.
+//
+class ScanHazardPtrPrintMatchingThreadsClosure : public ThreadClosure {
+ private:
+  JavaThread *_thread;
+ public:
+  ScanHazardPtrPrintMatchingThreadsClosure(JavaThread *thread) : _thread(thread) {}
+
+  virtual void do_thread(Thread *thread) {
+    assert_locked_or_safepoint(Threads_lock);
+
+    if (thread == NULL) return;
+    ThreadsList *current_list = thread->get_threads_hazard_ptr();
+    if (current_list == NULL) {
+      assert(thread->get_nested_threads_hazard_ptr() == NULL,
+             "cannot have a nested hazard ptr with a NULL regular hazard ptr");
+      return;
+    }
+    // If the hazard ptr is unverified, then ignore it.
+    if (Thread::is_hazard_ptr_tagged(current_list)) return;
+
+    // The current JavaThread has a hazard ptr (ThreadsList reference)
+    // which might be _smr_java_thread_list or it might be an older
+    // ThreadsList that has been removed but not freed. In either case,
+    // the hazard ptr is protecting all the JavaThreads on that
+    // ThreadsList, but we only care about matching a specific JavaThread.
+    JavaThreadIterator jti(current_list);
+    for (JavaThread *p = jti.first(); p != NULL; p = jti.next()) {
+      if (p == _thread) {
+        log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::smr_delete: thread1=" INTPTR_FORMAT " has a hazard pointer for thread2=" INTPTR_FORMAT, os::current_thread_id(), p2i(thread), p2i(_thread));
+        break;
+      }
+    }
+
+    // Any NestedThreadsLists are also protecting JavaThreads so
+    // check those also; the ThreadsLists may be different.
+    for (NestedThreadsList* node = thread->get_nested_threads_hazard_ptr();
+         node != NULL; node = node->next()) {
+      JavaThreadIterator jti(node->t_list());
+      for (JavaThread *p = jti.first(); p != NULL; p = jti.next()) {
+        if (p == _thread) {
+          log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::smr_delete: thread1=" INTPTR_FORMAT " has a nested hazard pointer for thread2=" INTPTR_FORMAT, os::current_thread_id(), p2i(thread), p2i(_thread));
+          return;
+        }
+      }
+    }
+  }
+};
+
 
 // 'entries + 1' so we always have at least one entry.
 ThreadsList::ThreadsList(int entries) : _length(entries), _threads(NEW_C_HEAP_ARRAY(JavaThread*, entries + 1, mtThread)), _next_list(NULL) {
   *(JavaThread**)(_threads + entries) = NULL;  // Make sure the extra entry is NULL.
 }
 
 ThreadsList::~ThreadsList() {
   FREE_C_HEAP_ARRAY(JavaThread*, _threads);
 }
 
+// Remove a JavaThread from a ThreadsList. The returned ThreadsList is a
+// new copy of the specified ThreadsList with the specified JavaThread
+// removed.
+ThreadsList *ThreadsList::remove_thread(ThreadsList* list, JavaThread* java_thread) {
+  assert(list->_length > 0, "sanity");
+
+  uint i = (uint)list->find_index_of_JavaThread(java_thread);
+  assert(i < list->_length, "did not find JavaThread on the list");
+  const uint index = i;
+  const uint new_length = list->_length - 1;
+  const uint head_length = index;
+  const uint tail_length = (new_length >= index) ? (new_length - index) : 0;
+  ThreadsList *const new_list = new ThreadsList(new_length);
+
+  if (head_length > 0) {
+    Copy::disjoint_words((HeapWord*)list->_threads, (HeapWord*)new_list->_threads, head_length);
+  }
+  if (tail_length > 0) {
+    Copy::disjoint_words((HeapWord*)list->_threads + index + 1, (HeapWord*)new_list->_threads + index, tail_length);
+  }
+
+  return new_list;
+}
+
+// Add a JavaThread to a ThreadsList. The returned ThreadsList is a
+// new copy of the specified ThreadsList with the specified JavaThread
+// appended to the end.
+ThreadsList *ThreadsList::add_thread(ThreadsList *list, JavaThread *java_thread) {
+  const uint index = list->_length;
+  const uint new_length = index + 1;
+  const uint head_length = index;
+  ThreadsList *const new_list = new ThreadsList(new_length);
+
+  if (head_length > 0) {
+    Copy::disjoint_words((HeapWord*)list->_threads, (HeapWord*)new_list->_threads, head_length);
+  }
+  *(JavaThread**)(new_list->_threads + index) = java_thread;
+
+  return new_list;
+}
+
+int ThreadsList::find_index_of_JavaThread(JavaThread *target) {
+  if (target == NULL) {
+    return -1;
+  }
+  for (uint i = 0; i < length(); i++) {
+    if (target == thread_at(i)) {
+      return (int)i;
+    }
+  }
+  return -1;
+}
+
+JavaThread* ThreadsList::find_JavaThread_from_java_tid(jlong java_tid) const {
+  for (uint i = 0; i < length(); i++) {
+    JavaThread* thread = thread_at(i);
+    oop tobj = thread->threadObj();
+    // Ignore the thread if it hasn't run yet, has exited
+    // or is starting to exit.
+    if (tobj != NULL && !thread->is_exiting() &&
+        java_tid == java_lang_Thread::thread_id(tobj)) {
+      // found a match
+      return thread;
+    }
+  }
+  return NULL;
+}
+
+bool ThreadsList::includes(const JavaThread * const p) const {
+  if (p == NULL) {
+    return false;
+  }
+  for (uint i = 0; i < length(); i++) {
+    if (thread_at(i) == p) {
+      return true;
+    }
+  }
+  return false;
+}
+
 ThreadsListSetter::~ThreadsListSetter() {
   if (_target_needs_release) {
     // The hazard ptr in the target needs to be released.
-    Threads::release_stable_list(_target);
+    ThreadsSMRSupport::release_stable_list(_target);
   }
 }
 
 void ThreadsListSetter::set() {
   assert(_target->get_threads_hazard_ptr() == NULL, "hazard ptr should not already be set");
-  (void) Threads::acquire_stable_list(_target, /* is_ThreadsListSetter */ true);
+  (void) ThreadsSMRSupport::acquire_stable_list(_target, /* is_ThreadsListSetter */ true);
   _target_needs_release = true;
 }
 
-ThreadsListHandle::ThreadsListHandle(Thread *self) : _list(Threads::acquire_stable_list(self, /* is_ThreadsListSetter */ false)), _self(self) {
+ThreadsListHandle::ThreadsListHandle(Thread *self) : _list(ThreadsSMRSupport::acquire_stable_list(self, /* is_ThreadsListSetter */ false)), _self(self) {
   assert(self == Thread::current(), "sanity check");
   if (EnableThreadSMRStatistics) {
     _timer.start();
   }
 }
 
 ThreadsListHandle::~ThreadsListHandle() {
-  Threads::release_stable_list(_self);
+  ThreadsSMRSupport::release_stable_list(_self);
   if (EnableThreadSMRStatistics) {
     _timer.stop();
     uint millis = (uint)_timer.milliseconds();
-    Threads::inc_smr_tlh_cnt();
-    Threads::add_smr_tlh_times(millis);
-    Threads::update_smr_tlh_time_max(millis);
+    ThreadsSMRSupport::inc_smr_tlh_cnt();
+    ThreadsSMRSupport::add_smr_tlh_times(millis);
+    ThreadsSMRSupport::update_smr_tlh_time_max(millis);
   }
 }
 
 // Convert an internal thread reference to a JavaThread found on the
 // associated ThreadsList. This ThreadsListHandle "protects" the

@@ -117,5 +530,505 @@
   // Return a live JavaThread that is "protected" by the
   // ThreadsListHandle in the caller.
   *jt_pp = java_thread;
   return true;
 }
+
+// Acquire a stable ThreadsList.
+//
+ThreadsList *ThreadsSMRSupport::acquire_stable_list(Thread *self, bool is_ThreadsListSetter) {
+  assert(self != NULL, "sanity check");
+  // acquire_stable_list_nested_path() will grab the Threads_lock
+  // so let's make sure the ThreadsListHandle is in a safe place.
+  // ThreadsListSetter cannot make this check on this code path.
+  debug_only(if (!is_ThreadsListSetter && StrictSafepointChecks) self->check_for_valid_safepoint_state(/* potential_vm_operation */ false);)
+
+  if (self->get_threads_hazard_ptr() == NULL) {
+    // The typical case is first.
+    return acquire_stable_list_fast_path(self);
+  }
+
+  // The nested case is rare.
+  return acquire_stable_list_nested_path(self);
+}
+
+// Fast path (and lock free) way to acquire a stable ThreadsList.
+//
+ThreadsList *ThreadsSMRSupport::acquire_stable_list_fast_path(Thread *self) {
+  assert(self != NULL, "sanity check");
+  assert(self->get_threads_hazard_ptr() == NULL, "sanity check");
+  assert(self->get_nested_threads_hazard_ptr() == NULL,
+         "cannot have a nested hazard ptr with a NULL regular hazard ptr");
+
+  ThreadsList* threads;
+
+  // Stable recording of a hazard ptr for SMR. This code does not use
+  // locks so its use of the _smr_java_thread_list & _threads_hazard_ptr
+  // fields is racy relative to code that uses those fields with locks.
+  // OrderAccess and Atomic functions are used to deal with those races.
+  //
+  while (true) {
+    threads = get_smr_java_thread_list();
+
+    // Publish a tagged hazard ptr to denote that the hazard ptr is not
+    // yet verified as being stable. Due to the fence after the hazard
+    // ptr write, it will be sequentially consistent w.r.t. the
+    // sequentially consistent writes of the ThreadsList, even on
+    // non-multiple copy atomic machines where stores can be observed
+    // in different order from different observer threads.
+    ThreadsList* unverified_threads = Thread::tag_hazard_ptr(threads);
+    self->set_threads_hazard_ptr(unverified_threads);
+
+    // If _smr_java_thread_list has changed, we have lost a race with
+    // Threads::add() or Threads::remove() and have to try again.
+    if (get_smr_java_thread_list() != threads) {
+      continue;
+    }
+
+    // We try to remove the tag which will verify the hazard ptr as
+    // being stable. This exchange can race with a scanning thread
+    // which might invalidate the tagged hazard ptr to keep it from
+    // being followed to access JavaThread ptrs. If we lose the race,
+    // we simply retry. If we win the race, then the stable hazard
+    // ptr is officially published.
+    if (self->cmpxchg_threads_hazard_ptr(threads, unverified_threads) == unverified_threads) {
+      break;
+    }
+  }
+
+  // A stable hazard ptr has been published letting other threads know
+  // that the ThreadsList and the JavaThreads reachable from this list
+  // are protected and hence they should not be deleted until everyone
+  // agrees it is safe to do so.
+
+  return threads;
+}
+
+// Acquire a nested stable ThreadsList; this is rare so it uses
+// Threads_lock.
+//
+ThreadsList *ThreadsSMRSupport::acquire_stable_list_nested_path(Thread *self) {
+  assert(self != NULL, "sanity check");
+  assert(self->get_threads_hazard_ptr() != NULL,
+         "cannot have a NULL regular hazard ptr when acquiring a nested hazard ptr");
+
+  // The thread already has a hazard ptr (ThreadsList ref) so we need
+  // to create a nested ThreadsListHandle with the current ThreadsList
+  // since it might be different than our current hazard ptr. The need
+  // for a nested ThreadsListHandle is rare so we do this while holding
+  // the Threads_lock so we don't race with the scanning code; the code
+  // is so much simpler this way.
+
+  NestedThreadsList* node;
+  {
+    // Only grab the Threads_lock if we don't already own it.
+    MutexLockerEx ml(Threads_lock->owned_by_self() ? NULL : Threads_lock);
+    node = new NestedThreadsList(get_smr_java_thread_list());
+    // We insert at the front of the list to match up with the delete
+    // in release_stable_list().
+    node->set_next(self->get_nested_threads_hazard_ptr());
+    self->set_nested_threads_hazard_ptr(node);
+    if (EnableThreadSMRStatistics) {
+      self->inc_nested_threads_hazard_ptr_cnt();
+      if (self->nested_threads_hazard_ptr_cnt() > _smr_nested_thread_list_max) {
+        _smr_nested_thread_list_max = self->nested_threads_hazard_ptr_cnt();
+      }
+    }
+  }
+  log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::acquire_stable_list: add NestedThreadsList node containing ThreadsList=" INTPTR_FORMAT, os::current_thread_id(), p2i(node->t_list()));
+
+  return node->t_list();
+}
+
+// set_smr_delete_notify() and clear_smr_delete_notify() are called
+// under the protection of the smr_delete_lock, but we also use an
+// Atomic operation to ensure the memory update is seen earlier than
+// when the smr_delete_lock is dropped.
+//
+void ThreadsSMRSupport::clear_smr_delete_notify() {
+  Atomic::dec(&_smr_delete_notify);
+}
+
+// Return true if the specified JavaThread is protected by a hazard
+// pointer (ThreadsList reference). Otherwise, returns false.
+//
+bool ThreadsSMRSupport::is_a_protected_JavaThread(JavaThread *thread) {
+  assert_locked_or_safepoint(Threads_lock);
+
+  // Hash table size should be first power of two higher than twice
+  // the length of the Threads list.
+  int hash_table_size = MIN2((int)get_smr_java_thread_list()->length(), 32) << 1;
+  hash_table_size--;
+  hash_table_size |= hash_table_size >> 1;
+  hash_table_size |= hash_table_size >> 2;
+  hash_table_size |= hash_table_size >> 4;
+  hash_table_size |= hash_table_size >> 8;
+  hash_table_size |= hash_table_size >> 16;
+  hash_table_size++;
+
+  // Gather a hash table of the JavaThreads indirectly referenced by
+  // hazard ptrs.
+  ThreadScanHashtable *scan_table = new ThreadScanHashtable(hash_table_size);
+  ScanHazardPtrGatherProtectedThreadsClosure scan_cl(scan_table);
+  Threads::threads_do(&scan_cl);
+
+  bool thread_is_protected = false;
+  if (scan_table->has_entry((void*)thread)) {
+    thread_is_protected = true;
+  }
+  delete scan_table;
+  return thread_is_protected;
+}
+
+// Release a stable ThreadsList.
+//
+void ThreadsSMRSupport::release_stable_list(Thread *self) {
+  assert(self != NULL, "sanity check");
+  // release_stable_list_nested_path() will grab the Threads_lock
+  // so let's make sure the ThreadsListHandle is in a safe place.
+  debug_only(if (StrictSafepointChecks) self->check_for_valid_safepoint_state(/* potential_vm_operation */ false);)
+
+  if (self->get_nested_threads_hazard_ptr() == NULL) {
+    // The typical case is first.
+    release_stable_list_fast_path(self);
+    return;
+  }
+
+  // The nested case is rare.
+  release_stable_list_nested_path(self);
+}
+
+// Fast path way to release a stable ThreadsList. The release portion
+// is lock-free, but the wake up portion is not.
+//
+void ThreadsSMRSupport::release_stable_list_fast_path(Thread *self) {
+  assert(self != NULL, "sanity check");
+  assert(self->get_threads_hazard_ptr() != NULL, "sanity check");
+  assert(self->get_nested_threads_hazard_ptr() == NULL,
+         "cannot have a nested hazard ptr when releasing a regular hazard ptr");
+
+  // After releasing the hazard ptr, other threads may go ahead and
+  // free up some memory temporarily used by a ThreadsList snapshot.
+  self->set_threads_hazard_ptr(NULL);
+
+  // We use double-check locking to reduce traffic on the system
+  // wide smr_delete_lock.
+  if (ThreadsSMRSupport::smr_delete_notify()) {
+    // An exiting thread might be waiting in smr_delete(); we need to
+    // check with smr_delete_lock to be sure.
+    release_stable_list_wake_up((char *) "regular hazard ptr");
+  }
+}
+
+// Release a nested stable ThreadsList; this is rare so it uses
+// Threads_lock.
+//
+void ThreadsSMRSupport::release_stable_list_nested_path(Thread *self) {
+  assert(self != NULL, "sanity check");
+  assert(self->get_nested_threads_hazard_ptr() != NULL, "sanity check");
+  assert(self->get_threads_hazard_ptr() != NULL,
+         "must have a regular hazard ptr to have nested hazard ptrs");
+
+  // We have a nested ThreadsListHandle so we have to release it first.
+  // The need for a nested ThreadsListHandle is rare so we do this while
+  // holding the Threads_lock so we don't race with the scanning code;
+  // the code is so much simpler this way.
+
+  NestedThreadsList *node;
+  {
+    // Only grab the Threads_lock if we don't already own it.
+    MutexLockerEx ml(Threads_lock->owned_by_self() ? NULL : Threads_lock);
+    // We remove from the front of the list to match up with the insert
+    // in acquire_stable_list().
+    node = self->get_nested_threads_hazard_ptr();
+    self->set_nested_threads_hazard_ptr(node->next());
+    if (EnableThreadSMRStatistics) {
+      self->dec_nested_threads_hazard_ptr_cnt();
+    }
+  }
+
+  // An exiting thread might be waiting in smr_delete(); we need to
+  // check with smr_delete_lock to be sure.
+  release_stable_list_wake_up((char *) "nested hazard ptr");
+
+  log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::release_stable_list: delete NestedThreadsList node containing ThreadsList=" INTPTR_FORMAT, os::current_thread_id(), p2i(node->t_list()));
+
+  delete node;
+}
+
+// Wake up portion of the release stable ThreadsList protocol;
+// uses the smr_delete_lock().
+//
+void ThreadsSMRSupport::release_stable_list_wake_up(char *log_str) {
+  assert(log_str != NULL, "sanity check");
+
+  // Note: smr_delete_lock is held in smr_delete() for the entire
+  // hazard ptr search so that we do not lose this notify() if
+  // the exiting thread has to wait. That code path also holds
+  // Threads_lock (which was grabbed before smr_delete_lock) so that
+  // threads_do() can be called. This means the system can't start a
+  // safepoint which means this thread can't take too long to get to
+  // a safepoint because of being blocked on smr_delete_lock.
+  //
+  MonitorLockerEx ml(ThreadsSMRSupport::smr_delete_lock(), Monitor::_no_safepoint_check_flag);
+  if (ThreadsSMRSupport::smr_delete_notify()) {
+    // Notify any exiting JavaThreads that are waiting in smr_delete()
+    // that we've released a ThreadsList.
+    ml.notify_all();
+    log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::release_stable_list notified %s", os::current_thread_id(), log_str);
+  }
+}
+
+// See note for clear_smr_delete_notify().
+//
+void ThreadsSMRSupport::set_smr_delete_notify() {
+  Atomic::inc(&_smr_delete_notify);
+}
+
+// Safely delete a JavaThread when it is no longer in use by a
+// ThreadsListHandle.
+//
+void ThreadsSMRSupport::smr_delete(JavaThread *thread) {
+  assert(!Threads_lock->owned_by_self(), "sanity");
+
+  bool has_logged_once = false;
+  elapsedTimer timer;
+  if (EnableThreadSMRStatistics) {
+    timer.start();
+  }
+
+  while (true) {
+    {
+      // No safepoint check because this JavaThread is not on the
+      // Threads list.
+      MutexLockerEx ml(Threads_lock, Mutex::_no_safepoint_check_flag);
+      // Cannot use a MonitorLockerEx helper here because we have
+      // to drop the Threads_lock first if we wait.
+      ThreadsSMRSupport::smr_delete_lock()->lock_without_safepoint_check();
+      // Set the smr_delete_notify flag after we grab smr_delete_lock
+      // and before we scan hazard ptrs because we're doing
+      // double-check locking in release_stable_list().
+      ThreadsSMRSupport::set_smr_delete_notify();
+
+      if (!is_a_protected_JavaThread(thread)) {
+        // This is the common case.
+        ThreadsSMRSupport::clear_smr_delete_notify();
+        ThreadsSMRSupport::smr_delete_lock()->unlock();
+        break;
+      }
+      if (!has_logged_once) {
+        has_logged_once = true;
+        log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::smr_delete: thread=" INTPTR_FORMAT " is not deleted.", os::current_thread_id(), p2i(thread));
+        if (log_is_enabled(Debug, os, thread)) {
+          ScanHazardPtrPrintMatchingThreadsClosure scan_cl(thread);
+          Threads::threads_do(&scan_cl);
+        }
+      }
+    } // We have to drop the Threads_lock to wait or delete the thread
+
+    if (EnableThreadSMRStatistics) {
+      _smr_delete_lock_wait_cnt++;
+      if (_smr_delete_lock_wait_cnt > _smr_delete_lock_wait_max) {
+        _smr_delete_lock_wait_max = _smr_delete_lock_wait_cnt;
+      }
+    }
+    // Wait for a release_stable_list() call before we check again. No
+    // safepoint check, no timeout, and not as suspend equivalent flag
+    // because this JavaThread is not on the Threads list.
+    ThreadsSMRSupport::smr_delete_lock()->wait(Mutex::_no_safepoint_check_flag, 0,
+                                     !Mutex::_as_suspend_equivalent_flag);
+    if (EnableThreadSMRStatistics) {
+      _smr_delete_lock_wait_cnt--;
+    }
+
+    ThreadsSMRSupport::clear_smr_delete_notify();
+    ThreadsSMRSupport::smr_delete_lock()->unlock();
+    // Retry the whole scenario.
+  }
+
+  if (ThreadLocalHandshakes) {
+    // The thread is about to be deleted so cancel any handshake.
+    thread->cancel_handshake();
+  }
+
+  delete thread;
+  if (EnableThreadSMRStatistics) {
+    timer.stop();
+    uint millis = (uint)timer.milliseconds();
+    ThreadsSMRSupport::inc_smr_deleted_thread_cnt();
+    ThreadsSMRSupport::add_smr_deleted_thread_times(millis);
+    ThreadsSMRSupport::update_smr_deleted_thread_time_max(millis);
+  }
+
+  log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::smr_delete: thread=" INTPTR_FORMAT " is deleted.", os::current_thread_id(), p2i(thread));
+}
+
+bool ThreadsSMRSupport::smr_delete_notify() {
+  // Use load_acquire() in order to see any updates to _smr_delete_notify
+  // earlier than when smr_delete_lock is grabbed.
+  return (OrderAccess::load_acquire(&_smr_delete_notify) != 0);
+}
+
+// Safely free a ThreadsList after a Threads::add() or Threads::remove().
+// The specified ThreadsList may not get deleted during this call if it
+// is still in-use (referenced by a hazard ptr). Other ThreadsLists
+// in the chain may get deleted by this call if they are no longer in-use.
+void ThreadsSMRSupport::smr_free_list(ThreadsList* threads) {
+  assert_locked_or_safepoint(Threads_lock);
+
+  threads->set_next_list(_smr_to_delete_list);
+  _smr_to_delete_list = threads;
+  if (EnableThreadSMRStatistics) {
+    _smr_to_delete_list_cnt++;
+    if (_smr_to_delete_list_cnt > _smr_to_delete_list_max) {
+      _smr_to_delete_list_max = _smr_to_delete_list_cnt;
+    }
+  }
+
+  // Hash table size should be first power of two higher than twice the length of the ThreadsList
+  int hash_table_size = MIN2((int)get_smr_java_thread_list()->length(), 32) << 1;
+  hash_table_size--;
+  hash_table_size |= hash_table_size >> 1;
+  hash_table_size |= hash_table_size >> 2;
+  hash_table_size |= hash_table_size >> 4;
+  hash_table_size |= hash_table_size >> 8;
+  hash_table_size |= hash_table_size >> 16;
+  hash_table_size++;
+
+  // Gather a hash table of the current hazard ptrs:
+  ThreadScanHashtable *scan_table = new ThreadScanHashtable(hash_table_size);
+  ScanHazardPtrGatherThreadsListClosure scan_cl(scan_table);
+  Threads::threads_do(&scan_cl);
+
+  // Walk through the linked list of pending freeable ThreadsLists
+  // and free the ones that are not referenced from hazard ptrs.
+  ThreadsList* current = _smr_to_delete_list;
+  ThreadsList* prev = NULL;
+  ThreadsList* next = NULL;
+  bool threads_is_freed = false;
+  while (current != NULL) {
+    next = current->next_list();
+    if (!scan_table->has_entry((void*)current)) {
+      // This ThreadsList is not referenced by a hazard ptr.
+      if (prev != NULL) {
+        prev->set_next_list(next);
+      }
+      if (_smr_to_delete_list == current) {
+        _smr_to_delete_list = next;
+      }
+
+      log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::smr_free_list: threads=" INTPTR_FORMAT " is freed.", os::current_thread_id(), p2i(current));
+      if (current == threads) threads_is_freed = true;
+      delete current;
+      if (EnableThreadSMRStatistics) {
+        _smr_java_thread_list_free_cnt++;
+        _smr_to_delete_list_cnt--;
+      }
+    } else {
+      prev = current;
+    }
+    current = next;
+  }
+
+  if (!threads_is_freed) {
+    // Only report "is not freed" on the original call to
+    // smr_free_list() for this ThreadsList.
+    log_debug(thread, smr)("tid=" UINTX_FORMAT ": ThreadsSMRSupport::smr_free_list: threads=" INTPTR_FORMAT " is not freed.", os::current_thread_id(), p2i(threads));
+  }
+
+  delete scan_table;
+}
+
+// Log Threads class SMR info.
+void ThreadsSMRSupport::log_smr_statistics() {
+  LogTarget(Info, thread, smr) log;
+  if (log.is_enabled()) {
+    LogStream out(log);
+    print_smr_info_on(&out);
+  }
+}
+
+// Print Threads class SMR info.
+void ThreadsSMRSupport::print_smr_info_on(outputStream* st) {
+  // Only grab the Threads_lock if we don't already own it
+  // and if we are not reporting an error.
+  MutexLockerEx ml((Threads_lock->owned_by_self() || VMError::is_error_reported()) ? NULL : Threads_lock);
+
+  st->print_cr("Threads class SMR info:");
+  st->print_cr("_smr_java_thread_list=" INTPTR_FORMAT ", length=%u, "
+               "elements={", p2i(_smr_java_thread_list),
+               _smr_java_thread_list->length());
+  print_smr_info_elements_on(st, _smr_java_thread_list);
+  st->print_cr("}");
+  if (_smr_to_delete_list != NULL) {
+    st->print_cr("_smr_to_delete_list=" INTPTR_FORMAT ", length=%u, "
+                 "elements={", p2i(_smr_to_delete_list),
+                 _smr_to_delete_list->length());
+    print_smr_info_elements_on(st, _smr_to_delete_list);
+    st->print_cr("}");
+    for (ThreadsList *t_list = _smr_to_delete_list->next_list();
+         t_list != NULL; t_list = t_list->next_list()) {
+      st->print("next-> " INTPTR_FORMAT ", length=%u, "
+                "elements={", p2i(t_list), t_list->length());
+      print_smr_info_elements_on(st, t_list);
+      st->print_cr("}");
+    }
+  }
+  if (!EnableThreadSMRStatistics) {
+    return;
+  }
+  st->print_cr("_smr_java_thread_list_alloc_cnt=" UINT64_FORMAT ","
+               "_smr_java_thread_list_free_cnt=" UINT64_FORMAT ","
+               "_smr_java_thread_list_max=%u, "
+               "_smr_nested_thread_list_max=%u",
+               _smr_java_thread_list_alloc_cnt,
+               _smr_java_thread_list_free_cnt,
+               _smr_java_thread_list_max,
+               _smr_nested_thread_list_max);
+  if (_smr_tlh_cnt > 0) {
+    st->print_cr("_smr_tlh_cnt=%u"
+                 ", _smr_tlh_times=%u"
+                 ", avg_smr_tlh_time=%0.2f"
+                 ", _smr_tlh_time_max=%u",
+                 _smr_tlh_cnt, _smr_tlh_times,
+                 ((double) _smr_tlh_times / _smr_tlh_cnt),
+                 _smr_tlh_time_max);
+  }
+  if (_smr_deleted_thread_cnt > 0) {
+    st->print_cr("_smr_deleted_thread_cnt=%u"
+                 ", _smr_deleted_thread_times=%u"
+                 ", avg_smr_deleted_thread_time=%0.2f"
+                 ", _smr_deleted_thread_time_max=%u",
+                 _smr_deleted_thread_cnt, _smr_deleted_thread_times,
+                 ((double) _smr_deleted_thread_times / _smr_deleted_thread_cnt),
+                 _smr_deleted_thread_time_max);
+  }
+  st->print_cr("_smr_delete_lock_wait_cnt=%u, _smr_delete_lock_wait_max=%u",
+               _smr_delete_lock_wait_cnt, _smr_delete_lock_wait_max);
+  st->print_cr("_smr_to_delete_list_cnt=%u, _smr_to_delete_list_max=%u",
+               _smr_to_delete_list_cnt, _smr_to_delete_list_max);
+}
+
+// Print ThreadsList elements (4 per line).
+void ThreadsSMRSupport::print_smr_info_elements_on(outputStream* st,
+                                         ThreadsList* t_list) {
+  uint cnt = 0;
+  JavaThreadIterator jti(t_list);
+  for (JavaThread *jt = jti.first(); jt != NULL; jt = jti.next()) {
+    st->print(INTPTR_FORMAT, p2i(jt));
+    if (cnt < t_list->length() - 1) {
+      // Separate with comma or comma-space except for the last one.
+      if (((cnt + 1) % 4) == 0) {
+        // Four INTPTR_FORMAT fit on an 80 column line so end the
+        // current line with just a comma.
+        st->print_cr(",");
+      } else {
+        // Not the last one on the current line so use comma-space:
+        st->print(", ");
+      }
+    } else {
+      // Last one so just end the current line.
+      st->cr();
+    }
+    cnt++;
+  }
+}