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14433074
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0
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2013-01-21T05:30:06.753
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2013-01-21T05:36:00.120
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2013-01-21T05:36:00.120
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14432400
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2
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consider this program: <pre><code>volatile int state; Integer result; void succeed(Integer result) if(state==PENDING) vr0 this.result = result; w1 state = DONE; vw1 Integer peekResult() if(state==DONE) vr2 return result; r2 return null; </code></pre> If volatile read <code>vr2</code> sees <code>DONE</code>, it means it happens after volatile write <code>vw1</code>. So we have happens-before relationships: <code>w1 -> vw1 -> vr2 -> r2</code>. Therefore write <code>w1</code> is visible to read <code>r2</code>. However <code>succeed()</code> isn't thread safe, since <code>vr0</code> and <code>vw1</code> are not atomic. If we use CAS <pre><code>void succeed(Integer result) if( compareAndSet(state, PENDING, DONE) ) vr0+vw0 this.result = result; w1 </code></pre> it fixes the atomicity issue. However, now <code>w1</code> isn't necessarily visible to <code>r2</code>. The memory barrier effect of CAS is kind of like <pre><code>void succeed(Integer result) if(state==PENDING) vr0 state=DONE; vw0 this.result = result; w1 </code></pre> We have here <code>vw0 -> vr2 -> r2</code>, but <code>w1</code> is not on the chain, there is no <code>w1 -> r2</code> We must do the volatile write <code>state=DONE</code> after <code>w1</code> to establish the happens-before chain. <pre><code>void succeed(Integer result) if(state==PENDING) vr0 state=TMP; vw0 this.result = result; w1 state=DONE; vw1 </code></pre> or in CAS <pre><code>void succeed(Integer result) if( compareAndSet(state, PENDING, TMP) ) vr0+vw0 this.result = result; w1 state=DONE; vw1 </code></pre>
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