--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/hotspot/os/linux/waitBarrier_linux.cpp	Fri Jan 11 10:58:46 2019 +0100
@@ -0,0 +1,80 @@
+/*
+ * Copyright (c) 2019, 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
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#include "precompiled/precompiled.hpp"
+#include "runtime/orderAccess.hpp"
+#include "runtime/os.hpp"
+#include "waitBarrier_linux.hpp"
+#include <sys/syscall.h>
+#include <linux/futex.h>
+
+#define check_with_errno(check_type, cond, msg)                             \
+  do {                                                                      \
+    int err = errno;                                                        \
+    check_type(cond, "%s: error='%s' (errno=%s)", msg, os::strerror(err),   \
+               os::errno_name(err));                                        \
+} while (false)
+
+#define guarantee_with_errno(cond, msg) check_with_errno(guarantee, cond, msg)
+
+static int futex(volatile int *addr, int futex_op, int op_arg) {
+  return syscall(SYS_futex, addr, futex_op, op_arg, NULL, NULL, 0);
+}
+
+void LinuxWaitBarrier::arm(int barrier_tag) {
+  assert(_futex_barrier == 0, "Should not be already armed: "
+         "_futex_barrier=%d", _futex_barrier);
+  _futex_barrier = barrier_tag;
+  OrderAccess::fence();
+}
+
+void LinuxWaitBarrier::disarm() {
+  assert(_futex_barrier != 0, "Should be armed/non-zero.");
+  _futex_barrier = 0;
+  int s = futex(&_futex_barrier,
+                FUTEX_WAKE_PRIVATE,
+                INT_MAX /* wake a max of this many threads */);
+  guarantee_with_errno(s > -1, "futex FUTEX_WAKE failed");
+}
+
+void LinuxWaitBarrier::wait(int barrier_tag) {
+  assert(barrier_tag != 0, "Trying to wait on disarmed value");
+  if (barrier_tag == 0 ||
+      barrier_tag != _futex_barrier) {
+    OrderAccess::fence();
+    return;
+  }
+  do {
+    int s = futex(&_futex_barrier,
+                  FUTEX_WAIT_PRIVATE,
+                  barrier_tag /* should be this tag */);
+    guarantee_with_errno((s == 0) ||
+                         (s == -1 && errno == EAGAIN) ||
+                         (s == -1 && errno == EINTR),
+                         "futex FUTEX_WAIT failed");
+    // Return value 0: woken up, but re-check in case of spurious wakeup.
+    // Error EINTR: woken by signal, so re-check and re-wait if necessary.
+    // Error EAGAIN: we are already disarmed and so will pass the check.
+  } while (barrier_tag == _futex_barrier);
+}

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/hotspot/os/linux/waitBarrier_linux.hpp	Fri Jan 11 10:58:46 2019 +0100
@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) 2019, 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
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#ifndef OS_LINUX_WAITBARRIER_LINUX_HPP
+#define OS_LINUX_WAITBARRIER_LINUX_HPP
+
+#include "memory/allocation.hpp"
+
+class LinuxWaitBarrier : public CHeapObj<mtInternal> {
+  volatile int _futex_barrier;
+
+  // Prevent copying and assignment of LinuxWaitBarrier instances.
+  LinuxWaitBarrier(const LinuxWaitBarrier&);
+  LinuxWaitBarrier& operator=(const LinuxWaitBarrier&);
+
+ public:
+  LinuxWaitBarrier() : _futex_barrier(0) {};
+  ~LinuxWaitBarrier() {};
+
+  const char* description() { return "futex"; }
+
+  void arm(int barrier_tag);
+  void disarm();
+  void wait(int barrier_tag);
+};
+
+#endif // OS_LINUX_WAITBARRIER_LINUX_HPP

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/hotspot/share/utilities/waitBarrier.hpp	Fri Jan 11 10:58:46 2019 +0100
@@ -0,0 +1,135 @@
+/*
+ * Copyright (c) 2019, 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
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#ifndef SHARE_UTILITIES_WAITBARRIER_HPP
+#define SHARE_UTILITIES_WAITBARRIER_HPP
+
+#include "memory/allocation.hpp"
+#include "runtime/thread.hpp"
+#include "utilities/debug.hpp"
+#include "utilities/waitBarrier_generic.hpp"
+
+#if defined(LINUX)
+#include "waitBarrier_linux.hpp"
+typedef LinuxWaitBarrier WaitBarrierDefault;
+#else
+typedef GenericWaitBarrier WaitBarrierDefault;
+#endif
+
+// Platform independent WaitBarrier API.
+// An armed WaitBarrier prevents threads from advancing until the threads are
+// woken by calling disarm(). The barrier is armed by setting a non-zero value
+// - the tag. When the WaitBarrier is created, a thread is designated the owner
+// and is the thread that should arm and disarm the WaitBarrier. In debug builds
+// this is enforced.
+//
+// Expected Usage:
+//  - Arming thread:
+//     tag = ...;  // non-zero value
+//     barrier.arm(tag);
+//     <publish tag>
+//     <work>
+//     barrier.disarm();
+//
+//    - After arm(tag) returns any thread calling wait(tag) will block.
+//    - Calling disarm() guarantees any thread calling or that has wait(tag) will
+//      return. Either they will see the WaitBarrier as disarmed or they will be
+//      unblocked and eligible to execute again when disarm() returns.
+//    - After calling disarm() the barrier is ready to be re-armed with a new tag.
+//      (may not be re-armed with last used tag)
+//
+//  - Waiting threads
+//     wait(tag); // don't execute following code unless 'safe'
+//     <work>
+//
+//    - A call to wait(tag) will block if the barrier is armed with the value
+//      'tag'; else it will return immediately.
+//    - A blocked thread is eligible to execute again once the barrier is
+//      disarmed when disarm() has been called.
+//
+// It is a usage error to:
+//  - call arm on a barrier that is already armed
+//  - call disarm on a barrier that is not armed
+//  - arm with the same tag as last used
+// Usage errors are checked in debug builds but may be ignored otherwise.
+//
+// A primary goal of the WaitBarrier implementation is to wake all waiting
+// threads as fast, and as concurrently, as possible.
+//
+template <typename WaitBarrierImpl>
+class WaitBarrierType : public CHeapObj<mtInternal> {
+  WaitBarrierImpl _impl;
+
+  // Prevent copying and assignment of WaitBarrier instances.
+  WaitBarrierType(const WaitBarrierDefault&);
+  WaitBarrierType& operator=(const WaitBarrierDefault&);
+
+#ifdef ASSERT
+  int _last_arm_tag;
+  Thread* _owner;
+#endif
+
+ public:
+  WaitBarrierType(Thread* owner) : _impl() {
+#ifdef ASSERT
+    _last_arm_tag = 0;
+    _owner = owner;
+#endif
+  }
+  ~WaitBarrierType() {}
+
+  // Returns implementation description.
+  const char* description()    { return _impl.description(); }
+
+  // Guarantees any thread calling wait() with same tag will be blocked.
+  // Provides a trailing fence.
+  void arm(int barrier_tag) {
+#ifdef ASSERT
+    assert(_last_arm_tag != barrier_tag, "Re-arming with same tag");
+    _last_arm_tag = barrier_tag;
+    assert(_owner == Thread::current(), "Not owner thread");
+#endif
+    _impl.arm(barrier_tag);
+  }
+
+  // Guarantees any thread that called wait() will be awake when it returns.
+  // Provides a trailing fence.
+  void disarm() {
+    assert(_owner == Thread::current(), "Not owner thread");
+    _impl.disarm();
+  }
+
+  // Guarantees not to return until disarm() is called,
+  // if called with currently armed tag (otherwise returns immediately).
+  // Implementations must guarantee no spurious wakeups.
+  // Provides a trailing fence.
+  void wait(int barrier_tag) {
+    assert(_owner != Thread::current(), "Trying to wait with owner thread");
+    _impl.wait(barrier_tag);
+  }
+};
+
+typedef WaitBarrierType<WaitBarrierDefault> WaitBarrier;
+
+#endif // SHARE_UTILITIES_WAITBARRIER_HPP

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/hotspot/share/utilities/waitBarrier_generic.cpp	Fri Jan 11 10:58:46 2019 +0100
@@ -0,0 +1,94 @@
+/*
+ * Copyright (c) 2019, 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
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#include "precompiled.hpp"
+#include "runtime/atomic.hpp"
+#include "runtime/orderAccess.hpp"
+#include "runtime/os.hpp"
+#include "utilities/waitBarrier_generic.hpp"
+#include "utilities/spinYield.hpp"
+
+void GenericWaitBarrier::arm(int barrier_tag) {
+  assert(_barrier_tag == 0, "Already armed");
+  assert(_waiters == 0, "We left a thread hanging");
+  _barrier_tag = barrier_tag;
+  _waiters = 0;
+  OrderAccess::fence();
+}
+
+int GenericWaitBarrier::wake_if_needed() {
+  assert(_barrier_tag == 0, "Not disarmed");
+  int w = _waiters;
+  if (w == 0) {
+    // Load of _barrier_threads in caller must not pass the load of _waiters.
+    OrderAccess::loadload();
+    return 0;
+  }
+  assert(w > 0, "Bad counting");
+  // We need an exact count which never goes below zero,
+  // otherwise the semaphore may be signalled too many times.
+  if (Atomic::cmpxchg(w - 1, &_waiters, w) == w) {
+    _sem_barrier.signal();
+    return w - 1;
+  }
+  return w;
+}
+
+void GenericWaitBarrier::disarm() {
+  assert(_barrier_tag != 0, "Not armed");
+  _barrier_tag = 0;
+  // Loads of _barrier_threads/_waiters must not float above disarm store and
+  // disarm store must not sink below.
+  OrderAccess::fence();
+  int left;
+  SpinYield sp;
+  do {
+    left = GenericWaitBarrier::wake_if_needed();
+    if (left == 0 && _barrier_threads > 0) {
+      // There is no thread to wake but we still have barrier threads.
+      sp.wait();
+    }
+    // We must loop here until there are no waiters or potential waiters.
+  } while (left > 0 || _barrier_threads > 0);
+  // API specifies disarm() must provide a trailing fence.
+  OrderAccess::fence();
+}
+
+void GenericWaitBarrier::wait(int barrier_tag) {
+  assert(barrier_tag != 0, "Trying to wait on disarmed value");
+  if (barrier_tag != _barrier_tag) {
+    // API specifies wait() must provide a trailing fence.
+    OrderAccess::fence();
+    return;
+  }
+  Atomic::add(1, &_barrier_threads);
+  if (barrier_tag != 0 && barrier_tag == _barrier_tag) {
+    Atomic::add(1, &_waiters);
+    _sem_barrier.wait();
+    // We help out with posting, but we need to do so before we decrement the
+    // _barrier_threads otherwise we might wake threads up in next wait.
+    GenericWaitBarrier::wake_if_needed();
+  }
+  Atomic::add(-1, &_barrier_threads);
+}

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/hotspot/share/utilities/waitBarrier_generic.hpp	Fri Jan 11 10:58:46 2019 +0100
@@ -0,0 +1,59 @@
+/*
+ * Copyright (c) 2019, 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
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#ifndef SHARE_UTILITIES_WAITBARRIER_GENERIC_HPP
+#define SHARE_UTILITIES_WAITBARRIER_GENERIC_HPP
+
+#include "memory/allocation.hpp"
+#include "runtime/semaphore.hpp"
+
+// In addition to the barrier tag, it uses two counters to keep the semaphore
+// count correct and not leave any late thread waiting.
+class GenericWaitBarrier : public CHeapObj<mtInternal> {
+  volatile int _barrier_tag;
+  // The number of threads waiting on or about to wait on the semaphore.
+  volatile int _waiters;
+  // The number of threads in the wait path, before or after the tag check.
+  // These threads can become waiters.
+  volatile int _barrier_threads;
+  Semaphore _sem_barrier;
+
+  // Prevent copying and assignment of GenericWaitBarrier instances.
+  GenericWaitBarrier(const GenericWaitBarrier&);
+  GenericWaitBarrier& operator=(const GenericWaitBarrier&);
+
+  int wake_if_needed();
+
+ public:
+  GenericWaitBarrier() : _barrier_tag(0), _waiters(0), _barrier_threads(0), _sem_barrier(0) {}
+  ~GenericWaitBarrier() {}
+
+  const char* description() { return "semaphore"; }
+
+  void arm(int barrier_tag);
+  void disarm();
+  void wait(int barrier_tag);
+};
+
+#endif // SHARE_UTILITIES_WAITBARRIER_GENERIC_HPP

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/hotspot/gtest/utilities/test_waitBarrier.cpp	Fri Jan 11 10:58:46 2019 +0100
@@ -0,0 +1,152 @@
+/*
+ * Copyright (c) 2019, 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
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+#include "precompiled.hpp"
+#include "runtime/atomic.hpp"
+#include "runtime/orderAccess.hpp"
+#include "runtime/os.hpp"
+#include "utilities/spinYield.hpp"
+#include "utilities/waitBarrier.hpp"
+#include "threadHelper.inline.hpp"
+
+static volatile int wait_tag = 0;
+static volatile int valid_value = 0;
+
+template <typename WaitBarrierImpl>
+class WBThread : public JavaTestThread {
+public:
+  static volatile bool _exit;
+  WaitBarrierType<WaitBarrierImpl>* _wait_barrier;
+  Semaphore* _wrt_start;
+  volatile int _on_barrier;
+
+  WBThread(Semaphore* post, WaitBarrierType<WaitBarrierImpl>* wb, Semaphore* wrt_start)
+    : JavaTestThread(post), _wait_barrier(wb), _wrt_start(wrt_start) {};
+  virtual ~WBThread(){}
+  void main_run() {
+    _wrt_start->signal();
+    int vv, tag;
+    // Similar to how a JavaThread would stop in a safepoint.
+    while (!_exit) {
+      // Load the published tag.
+      tag = OrderAccess::load_acquire(&wait_tag);
+      // Publish the tag this thread is going to wait for.
+      OrderAccess::release_store(&_on_barrier, tag);
+      if (_on_barrier == 0) {
+        SpinPause();
+        continue;
+      }
+      OrderAccess::storeload(); // Loads in WB must not float up.
+      // Wait until we are woken.
+      _wait_barrier->wait(tag);
+      // Verify that we do not see an invalid value.
+      vv = OrderAccess::load_acquire(&valid_value);
+      ASSERT_EQ((vv & 0x1), 0);
+      OrderAccess::release_store(&_on_barrier, 0);
+    }
+  }
+};
+
+template <typename WaitBarrierImpl>
+volatile bool WBThread<WaitBarrierImpl>::_exit = false;
+
+template <typename WaitBarrierImpl>
+class WBArmerThread : public JavaTestThread {
+public:
+  WBArmerThread(Semaphore* post) : JavaTestThread(post) {
+  };
+  virtual ~WBArmerThread(){}
+  void main_run() {
+    static const int NUMBER_OF_READERS = 4;
+    Semaphore post;
+    Semaphore wrt_start;
+    WaitBarrierType<WaitBarrierImpl> wb(this);
+
+    WBThread<WaitBarrierImpl>* reader1 = new WBThread<WaitBarrierImpl>(&post, &wb, &wrt_start);
+    WBThread<WaitBarrierImpl>* reader2 = new WBThread<WaitBarrierImpl>(&post, &wb, &wrt_start);
+    WBThread<WaitBarrierImpl>* reader3 = new WBThread<WaitBarrierImpl>(&post, &wb, &wrt_start);
+    WBThread<WaitBarrierImpl>* reader4 = new WBThread<WaitBarrierImpl>(&post, &wb, &wrt_start);
+
+    reader1->doit();
+    reader2->doit();
+    reader3->doit();
+    reader4->doit();
+
+    int nw = NUMBER_OF_READERS;
+    while (nw > 0) {
+      wrt_start.wait();
+      --nw;
+    }
+    jlong stop_ms = os::javaTimeMillis() + 1000; // 1 seconds max test time
+    int next_tag = 1;
+    // Similar to how the VM thread would use a WaitBarrier in a safepoint.
+    while (stop_ms > os::javaTimeMillis()) {
+      // Arm next tag.
+      wb.arm(next_tag);
+      // Publish tag.
+      OrderAccess::release_store_fence(&wait_tag, next_tag);
+
+      // Wait until threads picked up new tag.
+      while (reader1->_on_barrier != wait_tag ||
+             reader2->_on_barrier != wait_tag ||
+             reader3->_on_barrier != wait_tag ||
+             reader4->_on_barrier != wait_tag) {
+        SpinPause();
+      }
+
+      // Set an invalid value.
+      OrderAccess::release_store(&valid_value, valid_value + 1); // odd
+      os::naked_yield();
+      // Set a valid value.
+      OrderAccess::release_store(&valid_value, valid_value + 1); // even
+      // Publish inactive tag.
+      OrderAccess::release_store_fence(&wait_tag, 0); // Stores in WB must not float up.
+      wb.disarm();
+
+      // Wait until threads done valid_value verification.
+      while (reader1->_on_barrier != 0 ||
+             reader2->_on_barrier != 0 ||
+             reader3->_on_barrier != 0 ||
+             reader4->_on_barrier != 0) {
+        SpinPause();
+      }
+      ++next_tag;
+    }
+    WBThread<WaitBarrierImpl>::_exit = true;
+    for (int i = 0; i < NUMBER_OF_READERS; i++) {
+      post.wait();
+    }
+  }
+};
+
+TEST_VM(WaitBarrier, default_wb) {
+  WBThread<WaitBarrierDefault>::_exit = false;
+  mt_test_doer<WBArmerThread<WaitBarrierDefault> >();
+}
+
+#if defined(LINUX)
+TEST_VM(WaitBarrier, generic_wb) {
+  WBThread<GenericWaitBarrier>::_exit = false;
+  mt_test_doer<WBArmerThread<GenericWaitBarrier> >();
+}
+#endif