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1 /* |
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2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
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3 * |
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4 * This code is free software; you can redistribute it and/or modify it |
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5 * under the terms of the GNU General Public License version 2 only, as |
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6 * published by the Free Software Foundation. Sun designates this |
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7 * particular file as subject to the "Classpath" exception as provided |
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8 * by Sun in the LICENSE file that accompanied this code. |
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9 * |
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10 * This code is distributed in the hope that it will be useful, but WITHOUT |
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11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
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12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
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13 * version 2 for more details (a copy is included in the LICENSE file that |
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14 * accompanied this code). |
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15 * |
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16 * You should have received a copy of the GNU General Public License version |
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17 * 2 along with this work; if not, write to the Free Software Foundation, |
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18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
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19 * |
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20 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, |
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21 * CA 95054 USA or visit www.sun.com if you need additional information or |
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22 * have any questions. |
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23 */ |
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24 |
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25 /* |
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26 * This file is available under and governed by the GNU General Public |
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27 * License version 2 only, as published by the Free Software Foundation. |
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28 * However, the following notice accompanied the original version of this |
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29 * file: |
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30 * |
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31 * Written by Doug Lea with assistance from members of JCP JSR-166 |
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32 * Expert Group and released to the public domain, as explained at |
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33 * http://creativecommons.org/licenses/publicdomain |
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34 */ |
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35 |
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36 package java.util.concurrent.atomic; |
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37 |
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38 /** |
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39 * An {@code AtomicMarkableReference} maintains an object reference |
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40 * along with a mark bit, that can be updated atomically. |
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41 * <p> |
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42 * <p> Implementation note. This implementation maintains markable |
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43 * references by creating internal objects representing "boxed" |
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44 * [reference, boolean] pairs. |
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45 * |
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46 * @since 1.5 |
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47 * @author Doug Lea |
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48 * @param <V> The type of object referred to by this reference |
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49 */ |
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50 public class AtomicMarkableReference<V> { |
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51 |
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52 private static class ReferenceBooleanPair<T> { |
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53 private final T reference; |
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54 private final boolean bit; |
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55 ReferenceBooleanPair(T r, boolean i) { |
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56 reference = r; bit = i; |
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57 } |
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58 } |
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59 |
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60 private final AtomicReference<ReferenceBooleanPair<V>> atomicRef; |
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61 |
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62 /** |
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63 * Creates a new {@code AtomicMarkableReference} with the given |
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64 * initial values. |
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65 * |
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66 * @param initialRef the initial reference |
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67 * @param initialMark the initial mark |
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68 */ |
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69 public AtomicMarkableReference(V initialRef, boolean initialMark) { |
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70 atomicRef = new AtomicReference<ReferenceBooleanPair<V>> (new ReferenceBooleanPair<V>(initialRef, initialMark)); |
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71 } |
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72 |
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73 /** |
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74 * Returns the current value of the reference. |
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75 * |
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76 * @return the current value of the reference |
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77 */ |
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78 public V getReference() { |
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79 return atomicRef.get().reference; |
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80 } |
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81 |
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82 /** |
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83 * Returns the current value of the mark. |
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84 * |
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85 * @return the current value of the mark |
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86 */ |
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87 public boolean isMarked() { |
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88 return atomicRef.get().bit; |
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89 } |
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90 |
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91 /** |
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92 * Returns the current values of both the reference and the mark. |
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93 * Typical usage is {@code boolean[1] holder; ref = v.get(holder); }. |
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94 * |
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95 * @param markHolder an array of size of at least one. On return, |
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96 * {@code markholder[0]} will hold the value of the mark. |
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97 * @return the current value of the reference |
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98 */ |
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99 public V get(boolean[] markHolder) { |
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100 ReferenceBooleanPair<V> p = atomicRef.get(); |
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101 markHolder[0] = p.bit; |
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102 return p.reference; |
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103 } |
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104 |
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105 /** |
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106 * Atomically sets the value of both the reference and mark |
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107 * to the given update values if the |
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108 * current reference is {@code ==} to the expected reference |
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109 * and the current mark is equal to the expected mark. |
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110 * |
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111 * <p>May <a href="package-summary.html#Spurious">fail spuriously</a> |
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112 * and does not provide ordering guarantees, so is only rarely an |
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113 * appropriate alternative to {@code compareAndSet}. |
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114 * |
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115 * @param expectedReference the expected value of the reference |
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116 * @param newReference the new value for the reference |
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117 * @param expectedMark the expected value of the mark |
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118 * @param newMark the new value for the mark |
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119 * @return true if successful |
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120 */ |
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121 public boolean weakCompareAndSet(V expectedReference, |
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122 V newReference, |
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123 boolean expectedMark, |
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124 boolean newMark) { |
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125 ReferenceBooleanPair<V> current = atomicRef.get(); |
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126 return expectedReference == current.reference && |
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127 expectedMark == current.bit && |
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128 ((newReference == current.reference && newMark == current.bit) || |
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129 atomicRef.weakCompareAndSet(current, |
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130 new ReferenceBooleanPair<V>(newReference, |
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131 newMark))); |
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132 } |
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133 |
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134 /** |
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135 * Atomically sets the value of both the reference and mark |
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136 * to the given update values if the |
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137 * current reference is {@code ==} to the expected reference |
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138 * and the current mark is equal to the expected mark. |
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139 * |
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140 * @param expectedReference the expected value of the reference |
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141 * @param newReference the new value for the reference |
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142 * @param expectedMark the expected value of the mark |
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143 * @param newMark the new value for the mark |
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144 * @return true if successful |
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145 */ |
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146 public boolean compareAndSet(V expectedReference, |
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147 V newReference, |
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148 boolean expectedMark, |
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149 boolean newMark) { |
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150 ReferenceBooleanPair<V> current = atomicRef.get(); |
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151 return expectedReference == current.reference && |
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152 expectedMark == current.bit && |
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153 ((newReference == current.reference && newMark == current.bit) || |
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154 atomicRef.compareAndSet(current, |
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155 new ReferenceBooleanPair<V>(newReference, |
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156 newMark))); |
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157 } |
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158 |
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159 /** |
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160 * Unconditionally sets the value of both the reference and mark. |
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161 * |
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162 * @param newReference the new value for the reference |
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163 * @param newMark the new value for the mark |
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164 */ |
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165 public void set(V newReference, boolean newMark) { |
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166 ReferenceBooleanPair<V> current = atomicRef.get(); |
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167 if (newReference != current.reference || newMark != current.bit) |
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168 atomicRef.set(new ReferenceBooleanPair<V>(newReference, newMark)); |
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169 } |
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170 |
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171 /** |
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172 * Atomically sets the value of the mark to the given update value |
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173 * if the current reference is {@code ==} to the expected |
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174 * reference. Any given invocation of this operation may fail |
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175 * (return {@code false}) spuriously, but repeated invocation |
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176 * when the current value holds the expected value and no other |
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177 * thread is also attempting to set the value will eventually |
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178 * succeed. |
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179 * |
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180 * @param expectedReference the expected value of the reference |
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181 * @param newMark the new value for the mark |
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182 * @return true if successful |
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183 */ |
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184 public boolean attemptMark(V expectedReference, boolean newMark) { |
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185 ReferenceBooleanPair<V> current = atomicRef.get(); |
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186 return expectedReference == current.reference && |
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187 (newMark == current.bit || |
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188 atomicRef.compareAndSet |
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189 (current, new ReferenceBooleanPair<V>(expectedReference, |
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190 newMark))); |
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191 } |
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192 } |