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Java example source code file (park.cpp)

This example Java source code file (park.cpp) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

allocateheap, arv, associate, atomic\:\:cmpxchg_ptr, freelist, list, null, parker, parker\:\:allocate, parker\:\:listlock, parkevent, parkevent\:\:operator, shouldnotreachhere, tail\-

The park.cpp Java example source code

/*
 * 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
 * questions.
 *
 */

#include "precompiled.hpp"
#include "runtime/thread.hpp"



// Lifecycle management for TSM ParkEvents.
// ParkEvents are type-stable (TSM).
// In our particular implementation they happen to be immortal.
//
// We manage concurrency on the FreeList with a CAS-based
// detach-modify-reattach idiom that avoids the ABA problems
// that would otherwise be present in a simple CAS-based
// push-pop implementation.   (push-one and pop-all)
//
// Caveat: Allocate() and Release() may be called from threads
// other than the thread associated with the Event!
// If we need to call Allocate() when running as the thread in
// question then look for the PD calls to initialize native TLS.
// Native TLS (Win32/Linux/Solaris) can only be initialized or
// accessed by the associated thread.
// See also pd_initialize().
//
// Note that we could defer associating a ParkEvent with a thread
// until the 1st time the thread calls park().  unpark() calls to
// an unprovisioned thread would be ignored.  The first park() call
// for a thread would allocate and associate a ParkEvent and return
// immediately.

volatile int ParkEvent::ListLock = 0 ;
ParkEvent * volatile ParkEvent::FreeList = NULL ;

ParkEvent * ParkEvent::Allocate (Thread * t) {
  // In rare cases -- JVM_RawMonitor* operations -- we can find t == null.
  ParkEvent * ev ;

  // Start by trying to recycle an existing but unassociated
  // ParkEvent from the global free list.
  for (;;) {
    ev = FreeList ;
    if (ev == NULL) break ;
    // 1: Detach - sequester or privatize the list
    // Tantamount to ev = Swap (&FreeList, NULL)
    if (Atomic::cmpxchg_ptr (NULL, &FreeList, ev) != ev) {
       continue ;
    }

    // We've detached the list.  The list in-hand is now
    // local to this thread.   This thread can operate on the
    // list without risk of interference from other threads.
    // 2: Extract -- pop the 1st element from the list.
    ParkEvent * List = ev->FreeNext ;
    if (List == NULL) break ;
    for (;;) {
        // 3: Try to reattach the residual list
        guarantee (List != NULL, "invariant") ;
        ParkEvent * Arv =  (ParkEvent *) Atomic::cmpxchg_ptr (List, &FreeList, NULL) ;
        if (Arv == NULL) break ;

        // New nodes arrived.  Try to detach the recent arrivals.
        if (Atomic::cmpxchg_ptr (NULL, &FreeList, Arv) != Arv) {
            continue ;
        }
        guarantee (Arv != NULL, "invariant") ;
        // 4: Merge Arv into List
        ParkEvent * Tail = List ;
        while (Tail->FreeNext != NULL) Tail = Tail->FreeNext ;
        Tail->FreeNext = Arv ;
    }
    break ;
  }

  if (ev != NULL) {
    guarantee (ev->AssociatedWith == NULL, "invariant") ;
  } else {
    // Do this the hard way -- materialize a new ParkEvent.
    // In rare cases an allocating thread might detach a long list --
    // installing null into FreeList -- and then stall or be obstructed.
    // A 2nd thread calling Allocate() would see FreeList == null.
    // The list held privately by the 1st thread is unavailable to the 2nd thread.
    // In that case the 2nd thread would have to materialize a new ParkEvent,
    // even though free ParkEvents existed in the system.  In this case we end up
    // with more ParkEvents in circulation than we need, but the race is
    // rare and the outcome is benign.  Ideally, the # of extant ParkEvents
    // is equal to the maximum # of threads that existed at any one time.
    // Because of the race mentioned above, segments of the freelist
    // can be transiently inaccessible.  At worst we may end up with the
    // # of ParkEvents in circulation slightly above the ideal.
    // Note that if we didn't have the TSM/immortal constraint, then
    // when reattaching, above, we could trim the list.
    ev = new ParkEvent () ;
    guarantee ((intptr_t(ev) & 0xFF) == 0, "invariant") ;
  }
  ev->reset() ;                     // courtesy to caller
  ev->AssociatedWith = t ;          // Associate ev with t
  ev->FreeNext       = NULL ;
  return ev ;
}

void ParkEvent::Release (ParkEvent * ev) {
  if (ev == NULL) return ;
  guarantee (ev->FreeNext == NULL      , "invariant") ;
  ev->AssociatedWith = NULL ;
  for (;;) {
    // Push ev onto FreeList
    // The mechanism is "half" lock-free.
    ParkEvent * List = FreeList ;
    ev->FreeNext = List ;
    if (Atomic::cmpxchg_ptr (ev, &FreeList, List) == List) break ;
  }
}

// Override operator new and delete so we can ensure that the
// least significant byte of ParkEvent addresses is 0.
// Beware that excessive address alignment is undesirable
// as it can result in D$ index usage imbalance as
// well as bank access imbalance on Niagara-like platforms,
// although Niagara's hash function should help.

void * ParkEvent::operator new (size_t sz) throw() {
  return (void *) ((intptr_t (AllocateHeap(sz + 256, mtInternal, CALLER_PC)) + 256) & -256) ;
}

void ParkEvent::operator delete (void * a) {
  // ParkEvents are type-stable and immortal ...
  ShouldNotReachHere();
}


// 6399321 As a temporary measure we copied & modified the ParkEvent::
// allocate() and release() code for use by Parkers.  The Parker:: forms
// will eventually be removed as we consolide and shift over to ParkEvents
// for both builtin synchronization and JSR166 operations.

volatile int Parker::ListLock = 0 ;
Parker * volatile Parker::FreeList = NULL ;

Parker * Parker::Allocate (JavaThread * t) {
  guarantee (t != NULL, "invariant") ;
  Parker * p ;

  // Start by trying to recycle an existing but unassociated
  // Parker from the global free list.
  for (;;) {
    p = FreeList ;
    if (p  == NULL) break ;
    // 1: Detach
    // Tantamount to p = Swap (&FreeList, NULL)
    if (Atomic::cmpxchg_ptr (NULL, &FreeList, p) != p) {
       continue ;
    }

    // We've detached the list.  The list in-hand is now
    // local to this thread.   This thread can operate on the
    // list without risk of interference from other threads.
    // 2: Extract -- pop the 1st element from the list.
    Parker * List = p->FreeNext ;
    if (List == NULL) break ;
    for (;;) {
        // 3: Try to reattach the residual list
        guarantee (List != NULL, "invariant") ;
        Parker * Arv =  (Parker *) Atomic::cmpxchg_ptr (List, &FreeList, NULL) ;
        if (Arv == NULL) break ;

        // New nodes arrived.  Try to detach the recent arrivals.
        if (Atomic::cmpxchg_ptr (NULL, &FreeList, Arv) != Arv) {
            continue ;
        }
        guarantee (Arv != NULL, "invariant") ;
        // 4: Merge Arv into List
        Parker * Tail = List ;
        while (Tail->FreeNext != NULL) Tail = Tail->FreeNext ;
        Tail->FreeNext = Arv ;
    }
    break ;
  }

  if (p != NULL) {
    guarantee (p->AssociatedWith == NULL, "invariant") ;
  } else {
    // Do this the hard way -- materialize a new Parker..
    // In rare cases an allocating thread might detach
    // a long list -- installing null into FreeList --and
    // then stall.  Another thread calling Allocate() would see
    // FreeList == null and then invoke the ctor.  In this case we
    // end up with more Parkers in circulation than we need, but
    // the race is rare and the outcome is benign.
    // Ideally, the # of extant Parkers is equal to the
    // maximum # of threads that existed at any one time.
    // Because of the race mentioned above, segments of the
    // freelist can be transiently inaccessible.  At worst
    // we may end up with the # of Parkers in circulation
    // slightly above the ideal.
    p = new Parker() ;
  }
  p->AssociatedWith = t ;          // Associate p with t
  p->FreeNext       = NULL ;
  return p ;
}


void Parker::Release (Parker * p) {
  if (p == NULL) return ;
  guarantee (p->AssociatedWith != NULL, "invariant") ;
  guarantee (p->FreeNext == NULL      , "invariant") ;
  p->AssociatedWith = NULL ;
  for (;;) {
    // Push p onto FreeList
    Parker * List = FreeList ;
    p->FreeNext = List ;
    if (Atomic::cmpxchg_ptr (p, &FreeList, List) == List) break ;
  }
}

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