/*

 * 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 "memory/allocation.hpp"

#include "memory/padded.hpp"

class Thread;

// The GlobalCounter provides a synchronization mechanism between threads for

// safe memory reclamation and other ABA problems. All readers must call

// critical_section_begin before reading the volatile data and

// critical_section_end afterwards. Such read-side critical sections may

// be properly nested. The write side must call write_synchronize

// before reclaming the memory. The read-path only does an uncontended store

// to a thread-local-storage and fence to stop any loads from floating up, thus

// light weight and wait-free. The write-side is more heavy since it must check

// all readers and wait until they have left the generation. (a system memory

// barrier can be used on write-side to remove fence in read-side,

// not implemented).

class GlobalCounter : public AllStatic {

 private:

  // Since do not know what we will end up next to in BSS, we make sure the

  // counter is on a seperate cacheline.

  struct PaddedCounter {

    DEFINE_PAD_MINUS_SIZE(0, DEFAULT_CACHE_LINE_SIZE, 0);

    volatile uintx _counter;

    DEFINE_PAD_MINUS_SIZE(1, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile uintx));

  };

  // The global counter

  static PaddedCounter _global_counter;

  // Bit 0 is active bit.

  static const uintx COUNTER_ACTIVE = 1;

  // Thus we increase counter by 2.

  static const uintx COUNTER_INCREMENT = 2;

  // The per thread scanning closure.

  class CounterThreadCheck;

 public:

  // The type of the critical section context passed from

  // critical_section_begin() to critical_section_end().

  typedef uintx CSContext;

  // Must be called before accessing the data.  The result must be passed

  // to the associated call to critical_section_end().  Acts as a full

  // memory barrier before the code within the critical section.

  static CSContext critical_section_begin(Thread *thread);

  // Must be called after finished accessing the data.  The context

  // must be the result of the associated initiating critical_section_begin().

  // Acts as a release memory barrier after the code within the critical

  // section.

  static void critical_section_end(Thread *thread, CSContext context);

  // Make the data inaccessible to readers before calling. When this call

  // returns it's safe to reclaim the data.  Acts as a full memory barrier.

  static void write_synchronize();

  // A scoped object for a read-side critical-section.

  class CriticalSection;

};