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POCause of race condition in producer/consumer stack
 

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    <p>I've tried to create a producer-consumer stack, based on notification events, that would allow a single thread to push data, and another thread to pop data.</p> <p>When the buffer is full/empty, one thread waits for another until it is able to continue.</p> <p>I'm detecting a race condition (the program breaks where I have marked <code>***ERROR HERE***</code>) but I don't understand why it can happen.</p> <p>How can <code>size</code> go higher than <code>capacity</code> in this program?</p> <pre><code>#include &lt;process.h&gt; #include &lt;cstdlib&gt; #include &lt;vector&gt; #include &lt;windows.h&gt; template&lt;typename T, typename Ax = std::allocator&lt;T&gt; &gt; class rwstack { // It is assumed that only ONE thread will push data // and only ONE thread will pop data. public: typedef T value_type; typedef Ax allocator_type; typedef rwstack&lt;value_type, allocator_type&gt; this_type; typedef std::vector&lt;value_type, allocator_type&gt; container_type; private: allocator_type allocator; value_type *items; size_t volatile count; size_t const capacity; HANDLE hEventNotEmpty, hEventNotFull; rwstack(const this_type &amp;other) { __debugbreak(); /*Don't allow*/ } public: rwstack(const size_t capacity = 4096) : allocator(allocator_type()), items(allocator.allocate(capacity, NULL)), count(0), capacity(capacity), hEventNotEmpty(CreateEvent(NULL, TRUE, FALSE, NULL)), hEventNotFull(CreateEvent(NULL, TRUE, TRUE, NULL)) { } virtual ~rwstack() // Not actually used in the example { CloseHandle(hEventNotEmpty); CloseHandle(hEventNotFull); for (size_t i = 0; i &lt; count; i++) { allocator.destroy(&amp;items[InterlockedDecrementSizeT(&amp;count) - i]); } allocator.deallocate(items, capacity); } value_type &amp;push(const value_type &amp;value) { const ULONG waitResult = WaitForSingleObject(hEventNotFull, INFINITE); if (waitResult != WAIT_OBJECT_0) { __debugbreak(); } const size_t newSize = InterlockedIncrementSizeT(&amp;count); try { if (newSize &gt; capacity) { __debugbreak(); } // ****ERROR HERE**** if (newSize &gt;= capacity) { ResetEvent(hEventNotFull); } allocator.construct(&amp;items[newSize - 1], value); SetEvent(hEventNotEmpty); return items[newSize - 1]; } catch (...) { InterlockedDecrementSizeT(&amp;count); throw; } } void pop(value_type *pValue = NULL) { const ULONG waitResult = WaitForSingleObject(hEventNotEmpty, INFINITE); if (waitResult != WAIT_OBJECT_0) { __debugbreak(); } const size_t newSize = InterlockedDecrementSizeT(&amp;count); try { if (newSize &gt; capacity) { __debugbreak(); } // ****ERROR HERE**** if (newSize &lt;= 0) { ResetEvent(hEventNotEmpty); } if (pValue != NULL) { *pValue = items[newSize]; } allocator.destroy(&amp;items[newSize]); SetEvent(hEventNotFull); } catch (...) { InterlockedIncrementSizeT(&amp;count); throw; } } }; static size_t InterlockedIncrementSizeT(size_t volatile *p) { #if _M_X64 return InterlockedIncrement64(reinterpret_cast&lt;long long volatile *&gt;(p)); #elif _M_IX86 return InterlockedIncrement(reinterpret_cast&lt;long volatile *&gt;(p)); #endif } static size_t InterlockedDecrementSizeT(size_t volatile *p) { #if _M_X64 return InterlockedDecrement64(reinterpret_cast&lt;long long volatile *&gt;(p)); #elif _M_IX86 return InterlockedDecrement(reinterpret_cast&lt;long volatile *&gt;(p)); #endif } </code></pre> <p>Test code:</p> <pre><code>typedef rwstack&lt;int&gt; TTestStack; void __cdecl testPush(void *context) { TTestStack::value_type v; for (;;) static_cast&lt;TTestStack *&gt;(context)-&gt;pop(&amp;v); } void __cdecl testPop(void *context) { for (TTestStack::value_type v = 0; ; v++) static_cast&lt;TTestStack *&gt;(context)-&gt;push(v); } int main() { TTestStack rw; HANDLE hThreads[2] = { reinterpret_cast&lt;HANDLE&gt;(_beginthread(&amp;testPush, 0, &amp;rw)), reinterpret_cast&lt;HANDLE&gt;(_beginthread(&amp;testPop, 0, &amp;rw)), }; const ULONG nThreads = sizeof(hThreads) / sizeof(*hThreads) WaitForMultipleObjects(nThreads, hThreads, TRUE, INFINITE); return 0; } </code></pre>
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    1. USMehrdad
    1. USMehrdad
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    1. This table or related slice is empty.
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      1. POCause of race condition in producer/consumer stack
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    2. VO
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      1. POCause of race condition in producer/consumer stack
      1. USWill Bickford
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      1. USJagannath
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    1. CODid you try it in the simplified case of setting capacity=1, and if so, do you still have the problem?
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      1. POCause of race condition in producer/consumer stack
      1. USHostileFork
    2. COJust want to be know, since you are working on windows you don't have access to ppl ?
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      1. POCause of race condition in producer/consumer stack
      1. USJagannath
    3. CO@HostileFork: Didn't get a chance to test it, but I'll try it the next time I across a problem like this.
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      1. POCause of race condition in producer/consumer stack
      1. USMehrdad
 

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