1 /*
2 * Copyright (c) 1997, 2010, 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 * Per-thread blocking support for JSR166. See the Java-level
26 * Documentation for rationale. Basically, park acts like wait, unpark
27 * like notify.
28 *
29 * 6271289 --
30 * To avoid errors where an os thread expires but the JavaThread still
31 * exists, Parkers are immortal (type-stable) and are recycled across
32 * new threads. This parallels the ParkEvent implementation.
33 * Because park-unpark allow spurious wakeups it is harmless if an
34 * unpark call unparks a new thread using the old Parker reference.
35 *
36 * In the future we'll want to think about eliminating Parker and using
37 * ParkEvent instead. There's considerable duplication between the two
38 * services.
39 *
40 */
41
42 class Parker : public os::PlatformParker {
43 private:
44 volatile int _counter ;
45 Parker * FreeNext ;
46 JavaThread * AssociatedWith ; // Current association
47
48 public:
49 Parker() : PlatformParker() {
50 _counter = 0 ;
51 FreeNext = NULL ;
52 AssociatedWith = NULL ;
53 }
54 protected:
55 ~Parker() { ShouldNotReachHere(); }
56 public:
57 // For simplicity of interface with Java, all forms of park (indefinite,
58 // relative, and absolute) are multiplexed into one call.
59 void park(bool isAbsolute, jlong time);
60 void unpark();
61
62 // Lifecycle operators
63 static Parker * Allocate (JavaThread * t) ;
64 static void Release (Parker * e) ;
65 private:
66 static Parker * volatile FreeList ;
67 static volatile int ListLock ;
68
69 };
70
71 /////////////////////////////////////////////////////////////
72 //
73 // ParkEvents are type-stable and immortal.
74 //
75 // Lifecycle: Once a ParkEvent is associated with a thread that ParkEvent remains
76 // associated with the thread for the thread's entire lifetime - the relationship is
77 // stable. A thread will be associated at most one ParkEvent. When the thread
78 // expires, the ParkEvent moves to the EventFreeList. New threads attempt to allocate from
79 // the EventFreeList before creating a new Event. Type-stability frees us from
80 // worrying about stale Event or Thread references in the objectMonitor subsystem.
81 // (A reference to ParkEvent is always valid, even though the event may no longer be associated
82 // with the desired or expected thread. A key aspect of this design is that the callers of
83 // park, unpark, etc must tolerate stale references and spurious wakeups).
84 //
85 // Only the "associated" thread can block (park) on the ParkEvent, although
86 // any other thread can unpark a reachable parkevent. Park() is allowed to
87 // return spuriously. In fact park-unpark a really just an optimization to
88 // avoid unbounded spinning and surrender the CPU to be a polite system citizen.
89 // A degenerate albeit "impolite" park-unpark implementation could simply return.
90 // See http://blogs.sun.com/dave for more details.
91 //
92 // Eventually I'd like to eliminate Events and ObjectWaiters, both of which serve as
93 // thread proxies, and simply make the THREAD structure type-stable and persistent.
94 // Currently, we unpark events associated with threads, but ideally we'd just
95 // unpark threads.
96 //
97 // The base-class, PlatformEvent, is platform-specific while the ParkEvent is
98 // platform-independent. PlatformEvent provides park(), unpark(), etc., and
99 // is abstract -- that is, a PlatformEvent should never be instantiated except
100 // as part of a ParkEvent.
101 // Equivalently we could have defined a platform-independent base-class that
102 // exported Allocate(), Release(), etc. The platform-specific class would extend
103 // that base-class, adding park(), unpark(), etc.
104 //
105 // A word of caution: The JVM uses 2 very similar constructs:
106 // 1. ParkEvent are used for Java-level "monitor" synchronization.
107 // 2. Parkers are used by JSR166-JUC park-unpark.
108 //
109 // We'll want to eventually merge these redundant facilities and use ParkEvent.
110
111
112 class ParkEvent : public os::PlatformEvent {
113 private:
114 ParkEvent * FreeNext ;
115
116 // Current association
117 Thread * AssociatedWith ;
118 intptr_t RawThreadIdentity ; // LWPID etc
119 volatile int Incarnation ;
120
121 // diagnostic : keep track of last thread to wake this thread.
122 // this is useful for construction of dependency graphs.
123 void * LastWaker ;
124
125 public:
126 // MCS-CLH list linkage and Native Mutex/Monitor
127 ParkEvent * volatile ListNext ;
128 ParkEvent * volatile ListPrev ;
129 volatile intptr_t OnList ;
130 volatile int TState ;
131 volatile int Notified ; // for native monitor construct
132 volatile int IsWaiting ; // Enqueued on WaitSet
133
134
135 private:
136 static ParkEvent * volatile FreeList ;
137 static volatile int ListLock ;
138
139 // It's prudent to mark the dtor as "private"
140 // ensuring that it's not visible outside the package.
141 // Unfortunately gcc warns about such usage, so
142 // we revert to the less desirable "protected" visibility.
143 // The other compilers accept private dtors.
144
145 protected: // Ensure dtor is never invoked
146 ~ParkEvent() { guarantee (0, "invariant") ; }
147
148 ParkEvent() : PlatformEvent() {
149 AssociatedWith = NULL ;
150 FreeNext = NULL ;
151 ListNext = NULL ;
152 ListPrev = NULL ;
153 OnList = 0 ;
154 TState = 0 ;
155 Notified = 0 ;
156 IsWaiting = 0 ;
157 }
158
159 // We use placement-new to force ParkEvent instances to be
160 // aligned on 256-byte address boundaries. This ensures that the least
161 // significant byte of a ParkEvent address is always 0.
162
163 void * operator new (size_t sz) ;
164 void operator delete (void * a) ;
165
166 public:
167 static ParkEvent * Allocate (Thread * t) ;
168 static void Release (ParkEvent * e) ;
169 } ;