1 /* 2 * Copyright (c) 1997, 2020, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. 8 * 9 * This code is distributed in the hope that it will be useful, but WITHOUT 10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 12 * version 2 for more details (a copy is included in the LICENSE file that 13 * accompanied this code). 14 * 15 * You should have received a copy of the GNU General Public License version 16 * 2 along with this work; if not, write to the Free Software Foundation, 17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 18 * 19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 20 * or visit www.oracle.com if you need additional information or have any 21 * questions. 22 * 23 */ 24 25 #ifndef SHARE_RUNTIME_PARK_HPP 26 #define SHARE_RUNTIME_PARK_HPP 27 28 #include "utilities/debug.hpp" 29 #include "utilities/globalDefinitions.hpp" 30 /* 31 * Per-thread blocking support for JSR166. See the Java-level 32 * Documentation for rationale. Basically, park acts like wait, unpark 33 * like notify. 34 * 35 * 6271289 -- 36 * To avoid errors where an os thread expires but the JavaThread still 37 * exists, Parkers are immortal (type-stable) and are recycled across 38 * new threads. This parallels the ParkEvent implementation. 39 * Because park-unpark allow spurious wakeups it is harmless if an 40 * unpark call unparks a new thread using the old Parker reference. 41 * 42 * In the future we'll want to think about eliminating Parker and using 43 * ParkEvent instead. There's considerable duplication between the two 44 * services. 45 * 46 */ 47 48 class Parker : public os::PlatformParker { 49 private: 50 volatile int _counter ; 51 Parker * FreeNext ; 52 JavaThread * AssociatedWith ; // Current association 53 54 public: Parker()55 Parker() : PlatformParker() { 56 _counter = 0 ; 57 FreeNext = NULL ; 58 AssociatedWith = NULL ; 59 } 60 protected: ~Parker()61 ~Parker() { ShouldNotReachHere(); } 62 public: 63 // For simplicity of interface with Java, all forms of park (indefinite, 64 // relative, and absolute) are multiplexed into one call. 65 void park(bool isAbsolute, jlong time); 66 void unpark(); 67 68 // Lifecycle operators 69 static Parker * Allocate (JavaThread * t) ; 70 static void Release (Parker * e) ; 71 private: 72 static Parker * volatile FreeList ; 73 static volatile int ListLock ; 74 75 }; 76 77 ///////////////////////////////////////////////////////////// 78 // 79 // ParkEvents are type-stable and immortal. 80 // 81 // Lifecycle: Once a ParkEvent is associated with a thread that ParkEvent remains 82 // associated with the thread for the thread's entire lifetime - the relationship is 83 // stable. A thread will be associated at most one ParkEvent. When the thread 84 // expires, the ParkEvent moves to the EventFreeList. New threads attempt to allocate from 85 // the EventFreeList before creating a new Event. Type-stability frees us from 86 // worrying about stale Event or Thread references in the objectMonitor subsystem. 87 // (A reference to ParkEvent is always valid, even though the event may no longer be associated 88 // with the desired or expected thread. A key aspect of this design is that the callers of 89 // park, unpark, etc must tolerate stale references and spurious wakeups). 90 // 91 // Only the "associated" thread can block (park) on the ParkEvent, although 92 // any other thread can unpark a reachable parkevent. Park() is allowed to 93 // return spuriously. In fact park-unpark a really just an optimization to 94 // avoid unbounded spinning and surrender the CPU to be a polite system citizen. 95 // A degenerate albeit "impolite" park-unpark implementation could simply return. 96 // See http://blogs.sun.com/dave for more details. 97 // 98 // Eventually I'd like to eliminate Events and ObjectWaiters, both of which serve as 99 // thread proxies, and simply make the THREAD structure type-stable and persistent. 100 // Currently, we unpark events associated with threads, but ideally we'd just 101 // unpark threads. 102 // 103 // The base-class, PlatformEvent, is platform-specific while the ParkEvent is 104 // platform-independent. PlatformEvent provides park(), unpark(), etc., and 105 // is abstract -- that is, a PlatformEvent should never be instantiated except 106 // as part of a ParkEvent. 107 // Equivalently we could have defined a platform-independent base-class that 108 // exported Allocate(), Release(), etc. The platform-specific class would extend 109 // that base-class, adding park(), unpark(), etc. 110 // 111 // A word of caution: The JVM uses 2 very similar constructs: 112 // 1. ParkEvent are used for Java-level "monitor" synchronization. 113 // 2. Parkers are used by JSR166-JUC park-unpark. 114 // 115 // We'll want to eventually merge these redundant facilities and use ParkEvent. 116 117 118 class ParkEvent : public os::PlatformEvent { 119 private: 120 ParkEvent * FreeNext ; 121 122 // Current association 123 Thread * AssociatedWith ; 124 125 public: 126 // MCS-CLH list linkage and Native Mutex/Monitor 127 ParkEvent * volatile ListNext ; 128 volatile int TState ; 129 volatile int Notified ; // for native monitor construct 130 131 private: 132 static ParkEvent * volatile FreeList ; 133 static volatile int ListLock ; 134 135 // It's prudent to mark the dtor as "private" 136 // ensuring that it's not visible outside the package. 137 // Unfortunately gcc warns about such usage, so 138 // we revert to the less desirable "protected" visibility. 139 // The other compilers accept private dtors. 140 141 protected: // Ensure dtor is never invoked ~ParkEvent()142 ~ParkEvent() { guarantee (0, "invariant") ; } 143 ParkEvent()144 ParkEvent() : PlatformEvent() { 145 AssociatedWith = NULL ; 146 FreeNext = NULL ; 147 ListNext = NULL ; 148 TState = 0 ; 149 Notified = 0 ; 150 } 151 152 // We use placement-new to force ParkEvent instances to be 153 // aligned on 256-byte address boundaries. This ensures that the least 154 // significant byte of a ParkEvent address is always 0. 155 156 void * operator new (size_t sz) throw(); 157 void operator delete (void * a) ; 158 159 public: 160 static ParkEvent * Allocate (Thread * t) ; 161 static void Release (ParkEvent * e) ; 162 } ; 163 164 #endif // SHARE_RUNTIME_PARK_HPP 165