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1 /*
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2 * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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4 *
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5 * This code is free software; you can redistribute it and/or modify it
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6 * under the terms of the GNU General Public License version 2 only, as
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7 * published by the Free Software Foundation.
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8 *
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9 * This code is distributed in the hope that it will be useful, but WITHOUT
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10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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12 * version 2 for more details (a copy is included in the LICENSE file that
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13 * accompanied this code).
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14 *
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15 * You should have received a copy of the GNU General Public License version
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16 * 2 along with this work; if not, write to the Free Software Foundation,
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17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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18 *
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19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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20 * or visit www.oracle.com if you need additional information or have any
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21 * questions.
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22 *
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23 */
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24 /*
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25 * Per-thread blocking support for JSR166. See the Java-level
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26 * Documentation for rationale. Basically, park acts like wait, unpark
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27 * like notify.
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28 *
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29 * 6271289 --
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30 * To avoid errors where an os thread expires but the JavaThread still
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31 * exists, Parkers are immortal (type-stable) and are recycled across
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32 * new threads. This parallels the ParkEvent implementation.
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33 * Because park-unpark allow spurious wakeups it is harmless if an
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34 * unpark call unparks a new thread using the old Parker reference.
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35 *
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36 * In the future we'll want to think about eliminating Parker and using
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37 * ParkEvent instead. There's considerable duplication between the two
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38 * services.
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39 *
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40 */
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41
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42 class Parker : public os::PlatformParker {
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43 private:
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44 volatile int _counter ;
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45 Parker * FreeNext ;
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46 JavaThread * AssociatedWith ; // Current association
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47
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48 public:
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49 Parker() : PlatformParker() {
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50 _counter = 0 ;
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51 FreeNext = NULL ;
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52 AssociatedWith = NULL ;
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53 }
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54 protected:
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55 ~Parker() { ShouldNotReachHere(); }
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56 public:
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57 // For simplicity of interface with Java, all forms of park (indefinite,
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58 // relative, and absolute) are multiplexed into one call.
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59 void park(bool isAbsolute, jlong time);
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60 void unpark();
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61
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62 // Lifecycle operators
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63 static Parker * Allocate (JavaThread * t) ;
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64 static void Release (Parker * e) ;
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65 private:
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66 static Parker * volatile FreeList ;
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67 static volatile int ListLock ;
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68
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69 };
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70
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71 /////////////////////////////////////////////////////////////
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72 //
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73 // ParkEvents are type-stable and immortal.
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74 //
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75 // Lifecycle: Once a ParkEvent is associated with a thread that ParkEvent remains
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76 // associated with the thread for the thread's entire lifetime - the relationship is
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77 // stable. A thread will be associated at most one ParkEvent. When the thread
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78 // expires, the ParkEvent moves to the EventFreeList. New threads attempt to allocate from
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79 // the EventFreeList before creating a new Event. Type-stability frees us from
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80 // worrying about stale Event or Thread references in the objectMonitor subsystem.
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81 // (A reference to ParkEvent is always valid, even though the event may no longer be associated
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82 // with the desired or expected thread. A key aspect of this design is that the callers of
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83 // park, unpark, etc must tolerate stale references and spurious wakeups).
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84 //
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85 // Only the "associated" thread can block (park) on the ParkEvent, although
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86 // any other thread can unpark a reachable parkevent. Park() is allowed to
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87 // return spuriously. In fact park-unpark a really just an optimization to
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88 // avoid unbounded spinning and surrender the CPU to be a polite system citizen.
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89 // A degenerate albeit "impolite" park-unpark implementation could simply return.
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90 // See http://blogs.sun.com/dave for more details.
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91 //
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92 // Eventually I'd like to eliminate Events and ObjectWaiters, both of which serve as
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93 // thread proxies, and simply make the THREAD structure type-stable and persistent.
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94 // Currently, we unpark events associated with threads, but ideally we'd just
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95 // unpark threads.
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96 //
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97 // The base-class, PlatformEvent, is platform-specific while the ParkEvent is
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98 // platform-independent. PlatformEvent provides park(), unpark(), etc., and
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99 // is abstract -- that is, a PlatformEvent should never be instantiated except
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100 // as part of a ParkEvent.
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101 // Equivalently we could have defined a platform-independent base-class that
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102 // exported Allocate(), Release(), etc. The platform-specific class would extend
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103 // that base-class, adding park(), unpark(), etc.
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104 //
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105 // A word of caution: The JVM uses 2 very similar constructs:
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106 // 1. ParkEvent are used for Java-level "monitor" synchronization.
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107 // 2. Parkers are used by JSR166-JUC park-unpark.
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108 //
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109 // We'll want to eventually merge these redundant facilities and use ParkEvent.
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110
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111
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112 class ParkEvent : public os::PlatformEvent {
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113 private:
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114 ParkEvent * FreeNext ;
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115
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116 // Current association
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117 Thread * AssociatedWith ;
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118 intptr_t RawThreadIdentity ; // LWPID etc
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119 volatile int Incarnation ;
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120
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121 // diagnostic : keep track of last thread to wake this thread.
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122 // this is useful for construction of dependency graphs.
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123 void * LastWaker ;
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124
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125 public:
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126 // MCS-CLH list linkage and Native Mutex/Monitor
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127 ParkEvent * volatile ListNext ;
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128 ParkEvent * volatile ListPrev ;
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129 volatile intptr_t OnList ;
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130 volatile int TState ;
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131 volatile int Notified ; // for native monitor construct
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132 volatile int IsWaiting ; // Enqueued on WaitSet
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133
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134
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135 private:
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136 static ParkEvent * volatile FreeList ;
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137 static volatile int ListLock ;
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138
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139 // It's prudent to mark the dtor as "private"
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140 // ensuring that it's not visible outside the package.
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141 // Unfortunately gcc warns about such usage, so
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142 // we revert to the less desirable "protected" visibility.
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143 // The other compilers accept private dtors.
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144
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145 protected: // Ensure dtor is never invoked
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146 ~ParkEvent() { guarantee (0, "invariant") ; }
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147
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148 ParkEvent() : PlatformEvent() {
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149 AssociatedWith = NULL ;
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150 FreeNext = NULL ;
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151 ListNext = NULL ;
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152 ListPrev = NULL ;
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153 OnList = 0 ;
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154 TState = 0 ;
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155 Notified = 0 ;
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156 IsWaiting = 0 ;
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157 }
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158
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159 // We use placement-new to force ParkEvent instances to be
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160 // aligned on 256-byte address boundaries. This ensures that the least
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161 // significant byte of a ParkEvent address is always 0.
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162
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163 void * operator new (size_t sz) ;
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164 void operator delete (void * a) ;
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165
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166 public:
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167 static ParkEvent * Allocate (Thread * t) ;
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168 static void Release (ParkEvent * e) ;
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169 } ;
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