1 /*
   2  * Copyright (c) 2018, 2019, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #include "precompiled.hpp"
  26 #include "memory/iterator.hpp"
  27 #include "runtime/atomic.hpp"
  28 #include "runtime/thread.hpp"
  29 #include "runtime/threadSMR.inline.hpp"
  30 #include "runtime/vmThread.hpp"
  31 #include "utilities/globalCounter.hpp"
  32 #include "utilities/spinYield.hpp"
  33 
  34 GlobalCounter::PaddedCounter GlobalCounter::_global_counter;
  35 
  36 class GlobalCounter::CounterThreadCheck : public ThreadClosure {
  37  private:
  38   uintx _gbl_cnt;
  39  public:
  40   CounterThreadCheck(uintx gbl_cnt) : _gbl_cnt(gbl_cnt) {}
  41   void do_thread(Thread* thread) {
  42     SpinYield yield;
  43     // Loops on this thread until it has exited the critical read section.
  44     while(true) {
  45       uintx cnt = Atomic::load_acquire(thread->get_rcu_counter());
  46       // This checks if the thread's counter is active. And if so is the counter
  47       // for a pre-existing reader (belongs to this grace period). A pre-existing
  48       // reader will have a lower counter than the global counter version for this
  49       // generation. If the counter is larger than the global counter version this
  50       //  is a new reader and we can continue.
  51       if (((cnt & COUNTER_ACTIVE) != 0) && (cnt - _gbl_cnt) > (max_uintx / 2)) {
  52         yield.wait();
  53       } else {
  54         break;
  55       }
  56     }
  57   }
  58 };
  59 
  60 void GlobalCounter::write_synchronize() {
  61   assert((*Thread::current()->get_rcu_counter() & COUNTER_ACTIVE) == 0x0, "must be outside a critcal section");
  62   // Atomic::add must provide fence since we have storeload dependency.
  63   uintx gbl_cnt = Atomic::add(&_global_counter._counter, COUNTER_INCREMENT);
  64 
  65   // Do all RCU threads.
  66   CounterThreadCheck ctc(gbl_cnt);
  67   for (JavaThreadIteratorWithHandle jtiwh; JavaThread *thread = jtiwh.next(); ) {
  68     ctc.do_thread(thread);
  69   }
  70   for (NonJavaThread::Iterator njti; !njti.end(); njti.step()) {
  71     ctc.do_thread(njti.current());
  72   }
  73 }