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1 /*
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2 * Copyright 1998-2006 Sun Microsystems, Inc. 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
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20 * CA 95054 USA or visit www.sun.com if you need additional information or
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21 * have any questions.
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22 *
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23 */
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24
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25 // The SplitWord construct allows us to colocate the contention queue
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26 // (cxq) with the lock-byte. The queue elements are ParkEvents, which are
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27 // always aligned on 256-byte addresses - the least significant byte of
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28 // a ParkEvent is always 0. Colocating the lock-byte with the queue
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29 // allows us to easily avoid what would otherwise be a race in lock()
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30 // if we were to use two completely separate fields for the contention queue
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31 // and the lock indicator. Specifically, colocation renders us immune
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32 // from the race where a thread might enqueue itself in the lock() slow-path
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33 // immediately after the lock holder drops the outer lock in the unlock()
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34 // fast-path.
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35 //
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36 // Colocation allows us to use a fast-path unlock() form that uses
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37 // A MEMBAR instead of a CAS. MEMBAR has lower local latency than CAS
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38 // on many platforms.
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39 //
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40 // See:
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41 // + http://blogs.sun.com/dave/entry/biased_locking_in_hotspot
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42 // + http://blogs.sun.com/dave/resource/synchronization-public2.pdf
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43 //
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44 // Note that we're *not* using word-tearing the classic sense.
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45 // The lock() fast-path will CAS the lockword and the unlock()
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46 // fast-path will store into the lock-byte colocated within the lockword.
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47 // We depend on the fact that all our reference platforms have
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48 // coherent and atomic byte accesses. More precisely, byte stores
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49 // interoperate in a safe, sane, and expected manner with respect to
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50 // CAS, ST and LDs to the full-word containing the byte.
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51 // If you're porting HotSpot to a platform where that isn't the case
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52 // then you'll want change the unlock() fast path from:
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53 // STB;MEMBAR #storeload; LDN
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54 // to a full-word CAS of the lockword.
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55
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56
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57 union SplitWord { // full-word with separately addressable LSB
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58 volatile intptr_t FullWord ;
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59 volatile void * Address ;
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60 volatile jbyte Bytes [sizeof(intptr_t)] ;
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61 } ;
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62
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63 // Endian-ness ... index of least-significant byte in SplitWord.Bytes[]
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64 #ifdef AMD64 // little
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65 #define _LSBINDEX 0
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66 #else
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67 #if IA32 // little
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68 #define _LSBINDEX 0
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69 #else
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70 #ifdef SPARC // big
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71 #define _LSBINDEX (sizeof(intptr_t)-1)
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72 #else
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73 #error "unknown architecture"
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74 #endif
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75 #endif
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76 #endif
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77
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78 class ParkEvent ;
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79
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80 // See orderAccess.hpp. We assume throughout the VM that mutex lock and
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81 // try_lock do fence-lock-acquire, and that unlock does a release-unlock,
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82 // *in that order*. If their implementations change such that these
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83 // assumptions are violated, a whole lot of code will break.
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84
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85 class Monitor : public CHeapObj {
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86
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87 public:
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88 // A special lock: Is a lock where you are guaranteed not to block while you are
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89 // holding it, i.e., no vm operation can happen, taking other locks, etc.
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90 // NOTE: It is critical that the rank 'special' be the lowest (earliest)
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91 // (except for "event"?) for the deadlock dection to work correctly.
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92 // The rank native is only for use in Mutex's created by JVM_RawMonitorCreate,
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93 // which being external to the VM are not subject to deadlock detection.
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94 // The rank safepoint is used only for synchronization in reaching a
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95 // safepoint and leaving a safepoint. It is only used for the Safepoint_lock
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96 // currently. While at a safepoint no mutexes of rank safepoint are held
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97 // by any thread.
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98 // The rank named "leaf" is probably historical (and should
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99 // be changed) -- mutexes of this rank aren't really leaf mutexes
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100 // at all.
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101 enum lock_types {
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102 event,
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103 special,
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104 suspend_resume,
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105 leaf = suspend_resume + 2,
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106 safepoint = leaf + 10,
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107 barrier = safepoint + 1,
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108 nonleaf = barrier + 1,
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109 max_nonleaf = nonleaf + 900,
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110 native = max_nonleaf + 1
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111 };
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112
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113 // The WaitSet and EntryList linked lists are composed of ParkEvents.
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114 // I use ParkEvent instead of threads as ParkEvents are immortal and
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115 // type-stable, meaning we can safely unpark() a possibly stale
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116 // list element in the unlock()-path.
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117
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118 protected: // Monitor-Mutex metadata
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119 SplitWord _LockWord ; // Contention queue (cxq) colocated with Lock-byte
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120 enum LockWordBits { _LBIT=1 } ;
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121 Thread * volatile _owner; // The owner of the lock
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122 // Consider sequestering _owner on its own $line
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123 // to aid future synchronization mechanisms.
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124 ParkEvent * volatile _EntryList ; // List of threads waiting for entry
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125 ParkEvent * volatile _OnDeck ; // heir-presumptive
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126 volatile intptr_t _WaitLock [1] ; // Protects _WaitSet
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127 ParkEvent * volatile _WaitSet ; // LL of ParkEvents
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128 volatile bool _snuck; // Used for sneaky locking (evil).
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129 const char * _name; // Name of mutex
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130 int NotifyCount ; // diagnostic assist
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131 double pad [8] ; // avoid false sharing
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132
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133 // Debugging fields for naming, deadlock detection, etc. (some only used in debug mode)
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134 #ifndef PRODUCT
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135 bool _allow_vm_block;
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136 debug_only(int _rank;) // rank (to avoid/detect potential deadlocks)
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137 debug_only(Monitor * _next;) // Used by a Thread to link up owned locks
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138 debug_only(Thread* _last_owner;) // the last thread to own the lock
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139 debug_only(static bool contains(Monitor * locks, Monitor * lock);)
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140 debug_only(static Monitor * get_least_ranked_lock(Monitor * locks);)
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141 debug_only(Monitor * get_least_ranked_lock_besides_this(Monitor * locks);)
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142 #endif
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143
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144 void set_owner_implementation(Thread* owner) PRODUCT_RETURN;
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145 void check_prelock_state (Thread* thread) PRODUCT_RETURN;
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146 void check_block_state (Thread* thread) PRODUCT_RETURN;
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147
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148 // platform-dependent support code can go here (in os_<os_family>.cpp)
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149 public:
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150 enum {
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151 _no_safepoint_check_flag = true,
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152 _allow_vm_block_flag = true,
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153 _as_suspend_equivalent_flag = true
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154 };
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155
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156 enum WaitResults {
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157 CONDVAR_EVENT, // Wait returned because of condition variable notification
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158 INTERRUPT_EVENT, // Wait returned because waiting thread was interrupted
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159 NUMBER_WAIT_RESULTS
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160 };
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161
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162 private:
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163 int TrySpin (Thread * Self) ;
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164 int TryLock () ;
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165 int TryFast () ;
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166 int AcquireOrPush (ParkEvent * ev) ;
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167 void IUnlock (bool RelaxAssert) ;
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168 void ILock (Thread * Self) ;
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169 int IWait (Thread * Self, jlong timo);
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170 int ILocked () ;
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171
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172 protected:
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173 static void ClearMonitor (Monitor * m) ;
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174 Monitor() ;
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175
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176 public:
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177 Monitor(int rank, const char *name, bool allow_vm_block=false);
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178 ~Monitor();
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179
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180 // Wait until monitor is notified (or times out).
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181 // Defaults are to make safepoint checks, wait time is forever (i.e.,
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182 // zero), and not a suspend-equivalent condition. Returns true if wait
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183 // times out; otherwise returns false.
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184 bool wait(bool no_safepoint_check = !_no_safepoint_check_flag,
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185 long timeout = 0,
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186 bool as_suspend_equivalent = !_as_suspend_equivalent_flag);
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187 bool notify();
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188 bool notify_all();
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189
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190
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191 void lock(); // prints out warning if VM thread blocks
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192 void lock(Thread *thread); // overloaded with current thread
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193 void unlock();
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194 bool is_locked() const { return _owner != NULL; }
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195
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196 bool try_lock(); // Like lock(), but unblocking. It returns false instead
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197
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198 // Lock without safepoint check. Should ONLY be used by safepoint code and other code
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199 // that is guaranteed not to block while running inside the VM.
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200 void lock_without_safepoint_check();
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201 void lock_without_safepoint_check (Thread * Self) ;
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202
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203 // Current owner - not not MT-safe. Can only be used to guarantee that
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204 // the current running thread owns the lock
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205 Thread* owner() const { return _owner; }
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206 bool owned_by_self() const;
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207
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208 // Support for JVM_RawMonitorEnter & JVM_RawMonitorExit. These can be called by
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209 // non-Java thread. (We should really have a RawMonitor abstraction)
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210 void jvm_raw_lock();
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211 void jvm_raw_unlock();
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212 const char *name() const { return _name; }
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213
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214 void print_on_error(outputStream* st) const;
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215
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216 #ifndef PRODUCT
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217 void print_on(outputStream* st) const;
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218 void print() const { print_on(tty); }
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219 debug_only(int rank() const { return _rank; })
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220 bool allow_vm_block() { return _allow_vm_block; }
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221
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222 debug_only(Monitor *next() const { return _next; })
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223 debug_only(void set_next(Monitor *next) { _next = next; })
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224 #endif
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225
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226 void set_owner(Thread* owner) {
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227 #ifndef PRODUCT
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228 set_owner_implementation(owner);
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229 debug_only(void verify_Monitor(Thread* thr));
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230 #else
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231 _owner = owner;
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232 #endif
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233 }
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234
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235 };
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236
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237 // Normally we'd expect Monitor to extend Mutex in the sense that a monitor
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238 // constructed from pthreads primitives might extend a mutex by adding
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239 // a condvar and some extra metadata. In fact this was the case until J2SE7.
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240 //
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241 // Currently, however, the base object is a monitor. Monitor contains all the
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242 // logic for wait(), notify(), etc. Mutex extends monitor and restricts the
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243 // visiblity of wait(), notify(), and notify_all().
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244 //
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245 // Another viable alternative would have been to have Monitor extend Mutex and
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246 // implement all the normal mutex and wait()-notify() logic in Mutex base class.
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247 // The wait()-notify() facility would be exposed via special protected member functions
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248 // (e.g., _Wait() and _Notify()) in Mutex. Monitor would extend Mutex and expose wait()
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249 // as a call to _Wait(). That is, the public wait() would be a wrapper for the protected
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250 // _Wait().
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251 //
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252 // An even better alternative is to simply eliminate Mutex:: and use Monitor:: instead.
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253 // After all, monitors are sufficient for Java-level synchronization. At one point in time
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254 // there may have been some benefit to having distinct mutexes and monitors, but that time
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255 // has past.
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256 //
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257 // The Mutex/Monitor design parallels that of Java-monitors, being based on
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258 // thread-specific park-unpark platform-specific primitives.
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259
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260
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261 class Mutex : public Monitor { // degenerate Monitor
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262 public:
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263 Mutex (int rank, const char *name, bool allow_vm_block=false);
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264 ~Mutex () ;
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265 private:
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266 bool notify () { ShouldNotReachHere(); return false; }
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267 bool notify_all() { ShouldNotReachHere(); return false; }
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268 bool wait (bool no_safepoint_check, long timeout, bool as_suspend_equivalent) {
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269 ShouldNotReachHere() ;
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270 return false ;
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271 }
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272 };
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273
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274 /*
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275 * Per-thread blocking support for JSR166. See the Java-level
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276 * Documentation for rationale. Basically, park acts like wait, unpark
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277 * like notify.
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278 *
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279 * 6271289 --
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280 * To avoid errors where an os thread expires but the JavaThread still
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281 * exists, Parkers are immortal (type-stable) and are recycled across
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282 * new threads. This parallels the ParkEvent implementation.
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283 * Because park-unpark allow spurious wakeups it is harmless if an
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284 * unpark call unparks a new thread using the old Parker reference.
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285 *
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286 * In the future we'll want to think about eliminating Parker and using
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287 * ParkEvent instead. There's considerable duplication between the two
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288 * services.
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289 *
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290 */
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291
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292 class Parker : public os::PlatformParker {
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293 private:
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294 volatile int _counter ;
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295 Parker * FreeNext ;
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296 JavaThread * AssociatedWith ; // Current association
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297
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298 public:
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299 Parker() : PlatformParker() {
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300 _counter = 0 ;
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301 FreeNext = NULL ;
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302 AssociatedWith = NULL ;
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303 }
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304 protected:
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305 ~Parker() { ShouldNotReachHere(); }
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306 public:
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307 // For simplicity of interface with Java, all forms of park (indefinite,
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308 // relative, and absolute) are multiplexed into one call.
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309 void park(bool isAbsolute, jlong time);
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310 void unpark();
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311
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312 // Lifecycle operators
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313 static Parker * Allocate (JavaThread * t) ;
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314 static void Release (Parker * e) ;
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315 private:
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316 static Parker * volatile FreeList ;
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317 static volatile int ListLock ;
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318 };
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