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1 /*
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2 * Copyright 1997-2007 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 direct lock/unlock calls do not force a collection if an unlock
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26 // decrements the count to zero. Avoid calling these if at all possible.
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27
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28 class GC_locker: public AllStatic {
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29 private:
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30 static volatile jint _jni_lock_count; // number of jni active instances
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31 static volatile jint _lock_count; // number of other active instances
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32 static volatile bool _needs_gc; // heap is filling, we need a GC
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33 // note: bool is typedef'd as jint
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34 static volatile bool _doing_gc; // unlock_critical() is doing a GC
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35
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36 // Accessors
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37 static bool is_jni_active() {
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38 return _jni_lock_count > 0;
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39 }
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40
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41 static void set_needs_gc() {
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42 assert(SafepointSynchronize::is_at_safepoint(),
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43 "needs_gc is only set at a safepoint");
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44 _needs_gc = true;
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45 }
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46
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47 static void clear_needs_gc() {
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48 assert_lock_strong(JNICritical_lock);
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49 _needs_gc = false;
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50 }
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51
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52 static void jni_lock() {
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53 Atomic::inc(&_jni_lock_count);
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54 CHECK_UNHANDLED_OOPS_ONLY(
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55 if (CheckUnhandledOops) { Thread::current()->_gc_locked_out_count++; })
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56 assert(Universe::heap() == NULL || !Universe::heap()->is_gc_active(),
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57 "locking failed");
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58 }
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59
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60 static void jni_unlock() {
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61 Atomic::dec(&_jni_lock_count);
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62 CHECK_UNHANDLED_OOPS_ONLY(
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63 if (CheckUnhandledOops) { Thread::current()->_gc_locked_out_count--; })
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64 }
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65
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66 static void jni_lock_slow();
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67 static void jni_unlock_slow();
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68
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69 public:
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70 // Accessors
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71 static bool is_active();
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72 static bool needs_gc() { return _needs_gc; }
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73 // Shorthand
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74 static bool is_active_and_needs_gc() { return is_active() && needs_gc();}
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75
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76 // Calls set_needs_gc() if is_active() is true. Returns is_active().
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77 static bool check_active_before_gc();
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78
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79 // Stalls the caller (who should not be in a jni critical section)
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80 // until needs_gc() clears. Note however that needs_gc() may be
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81 // set at a subsequent safepoint and/or cleared under the
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82 // JNICritical_lock, so the caller may not safely assert upon
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83 // return from this method that "!needs_gc()" since that is
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84 // not a stable predicate.
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85 static void stall_until_clear();
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86
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87 // Non-structured GC locking: currently needed for JNI. Use with care!
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88 static void lock();
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89 static void unlock();
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90
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91 // The following two methods are used for JNI critical regions.
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92 // If we find that we failed to perform a GC because the GC_locker
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93 // was active, arrange for one as soon as possible by allowing
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94 // all threads in critical regions to complete, but not allowing
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95 // other critical regions to be entered. The reasons for that are:
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96 // 1) a GC request won't be starved by overlapping JNI critical
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97 // region activities, which can cause unnecessary OutOfMemory errors.
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98 // 2) even if allocation requests can still be satisfied before GC locker
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99 // becomes inactive, for example, in tenured generation possibly with
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100 // heap expansion, those allocations can trigger lots of safepointing
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101 // attempts (ineffective GC attempts) and require Heap_lock which
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102 // slow down allocations tremendously.
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103 //
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104 // Note that critical regions can be nested in a single thread, so
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105 // we must allow threads already in critical regions to continue.
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106 //
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107 // JNI critical regions are the only participants in this scheme
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108 // because they are, by spec, well bounded while in a critical region.
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109 //
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110 // Each of the following two method is split into a fast path and a slow
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111 // path. JNICritical_lock is only grabbed in the slow path.
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112 // _needs_gc is initially false and every java thread will go
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113 // through the fast path (which does the same thing as the slow path
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114 // when _needs_gc is false). When GC happens at a safepoint,
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115 // GC_locker::is_active() is checked. Since there is no safepoint in the
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116 // fast path of lock_critical() and unlock_critical(), there is no race
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117 // condition between the fast path and GC. After _needs_gc is set at a
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118 // safepoint, every thread will go through the slow path after the safepoint.
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119 // Since after a safepoint, each of the following two methods is either
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120 // entered from the method entry and falls into the slow path, or is
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121 // resumed from the safepoints in the method, which only exist in the slow
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122 // path. So when _needs_gc is set, the slow path is always taken, till
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123 // _needs_gc is cleared.
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124 static void lock_critical(JavaThread* thread);
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125 static void unlock_critical(JavaThread* thread);
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126 };
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127
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128
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129 // A No_GC_Verifier object can be placed in methods where one assumes that
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130 // no garbage collection will occur. The destructor will verify this property
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131 // unless the constructor is called with argument false (not verifygc).
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132 //
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133 // The check will only be done in debug mode and if verifygc true.
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134
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135 class No_GC_Verifier: public StackObj {
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136 friend class Pause_No_GC_Verifier;
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137
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138 protected:
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139 bool _verifygc;
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140 unsigned int _old_invocations;
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141
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142 public:
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143 #ifdef ASSERT
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144 No_GC_Verifier(bool verifygc = true);
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145 ~No_GC_Verifier();
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146 #else
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147 No_GC_Verifier(bool verifygc = true) {}
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148 ~No_GC_Verifier() {}
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149 #endif
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150 };
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151
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152 // A Pause_No_GC_Verifier is used to temporarily pause the behavior
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153 // of a No_GC_Verifier object. If we are not in debug mode or if the
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154 // No_GC_Verifier object has a _verifygc value of false, then there
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155 // is nothing to do.
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156
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157 class Pause_No_GC_Verifier: public StackObj {
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158 private:
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159 No_GC_Verifier * _ngcv;
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160
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161 public:
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162 #ifdef ASSERT
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163 Pause_No_GC_Verifier(No_GC_Verifier * ngcv);
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164 ~Pause_No_GC_Verifier();
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165 #else
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166 Pause_No_GC_Verifier(No_GC_Verifier * ngcv) {}
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167 ~Pause_No_GC_Verifier() {}
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168 #endif
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169 };
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170
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171
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172 // A No_Safepoint_Verifier object will throw an assertion failure if
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173 // the current thread passes a possible safepoint while this object is
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174 // instantiated. A safepoint, will either be: an oop allocation, blocking
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175 // on a Mutex or JavaLock, or executing a VM operation.
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176 //
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177 // If StrictSafepointChecks is turned off, it degrades into a No_GC_Verifier
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178 //
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179 class No_Safepoint_Verifier : public No_GC_Verifier {
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180 friend class Pause_No_Safepoint_Verifier;
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181
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182 private:
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183 bool _activated;
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184 Thread *_thread;
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185 public:
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186 #ifdef ASSERT
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187 No_Safepoint_Verifier(bool activated = true, bool verifygc = true ) : No_GC_Verifier(verifygc) {
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188 _thread = Thread::current();
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189 if (_activated) {
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190 _thread->_allow_allocation_count++;
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191 _thread->_allow_safepoint_count++;
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192 }
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193 }
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194
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195 ~No_Safepoint_Verifier() {
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196 if (_activated) {
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197 _thread->_allow_allocation_count--;
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198 _thread->_allow_safepoint_count--;
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199 }
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200 }
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201 #else
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202 No_Safepoint_Verifier(bool activated = true, bool verifygc = true) : No_GC_Verifier(verifygc){}
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203 ~No_Safepoint_Verifier() {}
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204 #endif
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205 };
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206
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207 // A Pause_No_Safepoint_Verifier is used to temporarily pause the
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208 // behavior of a No_Safepoint_Verifier object. If we are not in debug
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209 // mode then there is nothing to do. If the No_Safepoint_Verifier
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210 // object has an _activated value of false, then there is nothing to
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211 // do for safepoint and allocation checking, but there may still be
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212 // something to do for the underlying No_GC_Verifier object.
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213
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214 class Pause_No_Safepoint_Verifier : public Pause_No_GC_Verifier {
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215 private:
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216 No_Safepoint_Verifier * _nsv;
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217
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218 public:
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219 #ifdef ASSERT
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220 Pause_No_Safepoint_Verifier(No_Safepoint_Verifier * nsv)
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221 : Pause_No_GC_Verifier(nsv) {
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222
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223 _nsv = nsv;
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224 if (_nsv->_activated) {
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225 _nsv->_thread->_allow_allocation_count--;
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226 _nsv->_thread->_allow_safepoint_count--;
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227 }
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228 }
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229
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230 ~Pause_No_Safepoint_Verifier() {
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231 if (_nsv->_activated) {
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232 _nsv->_thread->_allow_allocation_count++;
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233 _nsv->_thread->_allow_safepoint_count++;
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234 }
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235 }
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236 #else
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237 Pause_No_Safepoint_Verifier(No_Safepoint_Verifier * nsv)
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238 : Pause_No_GC_Verifier(nsv) {}
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239 ~Pause_No_Safepoint_Verifier() {}
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240 #endif
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241 };
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242
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243 // JRT_LEAF currently can be called from either _thread_in_Java or
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244 // _thread_in_native mode. In _thread_in_native, it is ok
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245 // for another thread to trigger GC. The rest of the JRT_LEAF
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246 // rules apply.
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247 class JRT_Leaf_Verifier : public No_Safepoint_Verifier {
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248 static bool should_verify_GC();
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249 public:
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250 #ifdef ASSERT
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251 JRT_Leaf_Verifier();
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252 ~JRT_Leaf_Verifier();
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253 #else
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254 JRT_Leaf_Verifier() {}
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255 ~JRT_Leaf_Verifier() {}
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256 #endif
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257 };
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258
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259 // A No_Alloc_Verifier object can be placed in methods where one assumes that
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260 // no allocation will occur. The destructor will verify this property
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261 // unless the constructor is called with argument false (not activated).
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262 //
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263 // The check will only be done in debug mode and if activated.
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264 // Note: this only makes sense at safepoints (otherwise, other threads may
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265 // allocate concurrently.)
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266
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267 class No_Alloc_Verifier : public StackObj {
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268 private:
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269 bool _activated;
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270
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271 public:
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272 #ifdef ASSERT
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273 No_Alloc_Verifier(bool activated = true) {
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274 _activated = activated;
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275 if (_activated) Thread::current()->_allow_allocation_count++;
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276 }
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277
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278 ~No_Alloc_Verifier() {
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279 if (_activated) Thread::current()->_allow_allocation_count--;
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280 }
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281 #else
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282 No_Alloc_Verifier(bool activated = true) {}
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283 ~No_Alloc_Verifier() {}
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284 #endif
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285 };
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