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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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  */

 #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 ;

   }

 }