/*

 * Copyright (c) 1997, 2010, 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) {

  return (void *) ((intptr_t (CHeapObj::operator new (sz + 256)) + 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 ;

  }

}