/*

 * Copyright (c) 1997, 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

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 */

#ifndef SHARE_RUNTIME_PARK_HPP

#define SHARE_RUNTIME_PARK_HPP

#include "utilities/debug.hpp"

#include "utilities/globalDefinitions.hpp"

/*

 * Per-thread blocking support for JSR166. See the Java-level

 * Documentation for rationale. Basically, park acts like wait, unpark

 * like notify.

 *

 * 6271289 --

 * To avoid errors where an os thread expires but the JavaThread still

 * exists, Parkers are immortal (type-stable) and are recycled across

 * new threads.  This parallels the ParkEvent implementation.

 * Because park-unpark allow spurious wakeups it is harmless if an

 * unpark call unparks a new thread using the old Parker reference.

 *

 * In the future we'll want to think about eliminating Parker and using

 * ParkEvent instead.  There's considerable duplication between the two

 * services.

 *

 */

class Parker : public os::PlatformParker {

private:

  volatile int _counter ;

  Parker * FreeNext ;

  JavaThread * AssociatedWith ; // Current association

public:

  Parker() : PlatformParker() {

    _counter       = 0 ;

    FreeNext       = NULL ;

    AssociatedWith = NULL ;

  }

protected:

  ~Parker() { ShouldNotReachHere(); }

public:

  // For simplicity of interface with Java, all forms of park (indefinite,

  // relative, and absolute) are multiplexed into one call.

  void park(bool isAbsolute, jlong time);

  void unpark();

  // Lifecycle operators

  static Parker * Allocate (JavaThread * t) ;

  static void Release (Parker * e) ;

private:

  static Parker * volatile FreeList ;

  static volatile int ListLock ;

};

/////////////////////////////////////////////////////////////

//

// ParkEvents are type-stable and immortal.

//

// Lifecycle: Once a ParkEvent is associated with a thread that ParkEvent remains

// associated with the thread for the thread's entire lifetime - the relationship is

// stable. A thread will be associated at most one ParkEvent.  When the thread

// expires, the ParkEvent moves to the EventFreeList.  New threads attempt to allocate from

// the EventFreeList before creating a new Event.  Type-stability frees us from

// worrying about stale Event or Thread references in the objectMonitor subsystem.

// (A reference to ParkEvent is always valid, even though the event may no longer be associated

// with the desired or expected thread.  A key aspect of this design is that the callers of

// park, unpark, etc must tolerate stale references and spurious wakeups).

//

// Only the "associated" thread can block (park) on the ParkEvent, although

// any other thread can unpark a reachable parkevent.  Park() is allowed to

// return spuriously.  In fact park-unpark a really just an optimization to

// avoid unbounded spinning and surrender the CPU to be a polite system citizen.

// A degenerate albeit "impolite" park-unpark implementation could simply return.

// See http://blogs.sun.com/dave for more details.

//

// Eventually I'd like to eliminate Events and ObjectWaiters, both of which serve as

// thread proxies, and simply make the THREAD structure type-stable and persistent.

// Currently, we unpark events associated with threads, but ideally we'd just

// unpark threads.

//

// The base-class, PlatformEvent, is platform-specific while the ParkEvent is

// platform-independent.  PlatformEvent provides park(), unpark(), etc., and

// is abstract -- that is, a PlatformEvent should never be instantiated except

// as part of a ParkEvent.

// Equivalently we could have defined a platform-independent base-class that

// exported Allocate(), Release(), etc.  The platform-specific class would extend

// that base-class, adding park(), unpark(), etc.

//

// A word of caution: The JVM uses 2 very similar constructs:

// 1. ParkEvent are used for Java-level "monitor" synchronization.

// 2. Parkers are used by JSR166-JUC park-unpark.

//

// We'll want to eventually merge these redundant facilities and use ParkEvent.

class ParkEvent : public os::PlatformEvent {

  private:

    ParkEvent * FreeNext ;

    // Current association

    Thread * AssociatedWith ;

  public:

    // MCS-CLH list linkage and Native Mutex/Monitor

    ParkEvent * volatile ListNext ;

    volatile intptr_t OnList ;

    volatile int TState ;

    volatile int Notified ;             // for native monitor construct

  private:

    static ParkEvent * volatile FreeList ;

    static volatile int ListLock ;

    // It's prudent to mark the dtor as "private"

    // ensuring that it's not visible outside the package.

    // Unfortunately gcc warns about such usage, so

    // we revert to the less desirable "protected" visibility.

    // The other compilers accept private dtors.

  protected:        // Ensure dtor is never invoked

    ~ParkEvent() { guarantee (0, "invariant") ; }

    ParkEvent() : PlatformEvent() {

       AssociatedWith = NULL ;

       FreeNext       = NULL ;

       ListNext       = NULL ;

       OnList         = 0 ;

       TState         = 0 ;

       Notified       = 0 ;

    }

    // We use placement-new to force ParkEvent instances to be

    // aligned on 256-byte address boundaries.  This ensures that the least

    // significant byte of a ParkEvent address is always 0.

    void * operator new (size_t sz) throw();

    void operator delete (void * a) ;

  public:

    static ParkEvent * Allocate (Thread * t) ;

    static void Release (ParkEvent * e) ;

} ;

#endif // SHARE_RUNTIME_PARK_HPP