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POconfused about spin lock
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copied!<p>I read spin lock code <a href="http://www.codeproject.com/KB/threads/spinlocks.aspx" rel="nofollow noreferrer">from here</a>, especially this part</p> <pre><code>inline void Enter(void) { int prev_s; do { prev_s = TestAndSet(&m_s, 0); if (m_s == 0 && prev_s == 1) { break; } // reluinquish current timeslice (can only // be used when OS available and // we do NOT want to 'spin') // HWSleep(0); } while (true); } </code></pre> <p>Why do we need to test two conditions m_s == 0 && prev_s == 1? I think just test prev_s == 1 should be enough. Any ideas?</p> <p>EDIT: version 2. should we fix in this way if there is a bug?</p> <pre><code>inline void Enter(void) { do { if (m_s == 0 && 1 == TestAndSet(&m_s, 0)) { break; } // reluinquish current timeslice (can only // be used when OS available and // we do NOT want to 'spin') // HWSleep(0); } while (true); } </code></pre> <p>EDIT: version 3. I think version 3 from functional level is correct, but performance is not good enough since each time we need to write, no read test ahead. Is my understanding correct?</p> <pre><code>inline void Enter(void) { do { if (1 == TestAndSet(&m_s, 0)) { break; } // reluinquish current timeslice (can only // be used when OS available and // we do NOT want to 'spin') // HWSleep(0); } while (true); } </code></pre> <p>@dragonfly, here is my bug fix version 4 (fixed a bug in version 2 as you pointed out), could you review whether it is correct please? Thanks!</p> <p>EDIT: version 4.</p> <pre><code>inline void Enter(void) { do { if (m_s == 1 && 1 == TestAndSet(&m_s, 0)) { break; } // reluinquish current timeslice (can only // be used when OS available and // we do NOT want to 'spin') // HWSleep(0); } while (true); } </code></pre>

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