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POIntel Inspector reports a data race in my spinlock implementation
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  1. POIntel Inspector reports a data race in my spinlock implementation
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    copied!<p>I made a very simple spinlock using the Interlocked functions in Windows and tested it on a dual-core CPU (two threads that increment a variable); </p> <p>The program seems to work OK (it gives the same result every time, which is not the case when no synchronization is used), but <em>Intel Parallel Inspector</em> says that there is a race condition at <em>value += j</em> (see the code below). The warning disappears when using Critical Sections instead of my SpinLock. </p> <p>Is my implementation of SpinLock correct or not ? It's really strange, because all the used operations are atomic and have the proper memory barriers and it shouldn't lead to race conditions.</p> <pre><code>class SpinLock { int *lockValue; SpinLock(int *value) : lockValue(value) { } void Lock() { while(InterlockedCompareExchange((volatile LONG*)lockValue, 1, 0) != 0) { WaitABit(); } } void Unlock() { InterlockedExchange((volatile LONG*)lockValue, 0); } }; </code></pre> <p>The test program: </p> <pre><code>static const int THREADS = 2; HANDLE completedEvents[THREADS]; int value = 0; int lock = 0; // Global. DWORD WINAPI TestThread(void *param) { HANDLE completed = (HANDLE)param; SpinLock testLock(&amp;lock); for(int i = 0;i &lt; 1000*20; i++) { for(int j = 0;j &lt; 10*10; j++) { // Add something to the variable. testLock.Lock(); value += j; testLock.Unlock(); } } SetEvent(completed); } int main() { for(int i = 0; i &lt; THREADS; i++) { completedEvents[i] = CreateEvent(NULL, true, false, NULL); } for(int i = 0; i &lt; THREADS; i++) { DWORD id; CreateThread(NULL, 0, TestThread, completedEvents[i], 0, &amp;id); } WaitForMultipleObjects(THREADS, completedEvents, true, INFINITE); cout&lt;&lt;value; } </code></pre>
 

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