1 /*
2 * Copyright (c) 1997, 2013, 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
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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
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23 */
24
25 #ifndef SHARE_VM_RUNTIME_PARK_HPP
26 #define SHARE_VM_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 intptr_t _counter ;
51 Parker * FreeNext ;
52 JavaThread * AssociatedWith ; // Current association
53
54 public:
55 Parker() : PlatformParker() {
56 _counter = 0 ;
57 FreeNext = NULL ;
58 AssociatedWith = NULL ;
59 }
60 protected:
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(JavaThread *thread);
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 intptr_t RawThreadIdentity ; // LWPID etc
125 volatile int Incarnation ;
126
127 // diagnostic : keep track of last thread to wake this thread.
128 // this is useful for construction of dependency graphs.
129 void * LastWaker ;
130
131 public:
132 // MCS-CLH list linkage and Native Mutex/Monitor
133 ParkEvent * volatile ListNext ;
134 ParkEvent * volatile ListPrev ;
135 volatile intptr_t OnList ;
136 volatile int TState ;
137 volatile int Notified ; // for native monitor construct
138 volatile int IsWaiting ; // Enqueued on WaitSet
139
140
141 private:
142 static ParkEvent * volatile FreeList ;
143 static volatile int ListLock ;
144
145 // It's prudent to mark the dtor as "private"
146 // ensuring that it's not visible outside the package.
147 // Unfortunately gcc warns about such usage, so
148 // we revert to the less desirable "protected" visibility.
149 // The other compilers accept private dtors.
150
151 protected: // Ensure dtor is never invoked
152 ~ParkEvent() { guarantee (0, "invariant") ; }
153
154 ParkEvent() : PlatformEvent() {
155 AssociatedWith = NULL ;
156 FreeNext = NULL ;
157 ListNext = NULL ;
158 ListPrev = NULL ;
159 OnList = 0 ;
160 TState = 0 ;
161 Notified = 0 ;
162 IsWaiting = 0 ;
163 }
164
165 // We use placement-new to force ParkEvent instances to be
166 // aligned on 256-byte address boundaries. This ensures that the least
167 // significant byte of a ParkEvent address is always 0.
168
169 void * operator new (size_t sz) throw();
170 void operator delete (void * a) ;
171
172 public:
173 static ParkEvent * Allocate (Thread * t) ;
174 static void Release (ParkEvent * e) ;
175 } ;
176
177 #endif // SHARE_VM_RUNTIME_PARK_HPP