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 /*

  * Copyright (c) 1997, 2013, 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

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  */

 #ifndef SHARE_VM_RUNTIME_PARK_HPP

 #define SHARE_VM_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 ;

     intptr_t RawThreadIdentity ;        // LWPID etc

     volatile int Incarnation ;

     // diagnostic : keep track of last thread to wake this thread.

     // this is useful for construction of dependency graphs.

     void * LastWaker ;

   public:

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

     ParkEvent * volatile ListNext ;

     ParkEvent * volatile ListPrev ;

     volatile intptr_t OnList ;

     volatile int TState ;

     volatile int Notified ;             // for native monitor construct

     volatile int IsWaiting ;            // Enqueued on WaitSet

   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 ;

        ListPrev       = NULL ;

        OnList         = 0 ;

        TState         = 0 ;

        Notified       = 0 ;

        IsWaiting      = 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_VM_RUNTIME_PARK_HPP