CODEWITHSUNDEEP
cmultithreadingc++11gcccompare-and-swap
Django
Django

Reputation: 103

__sync_bool_compare_and_swap not working

// created a copy of CNode and added the new value
    CNode *copyCNode = new CNode;
    //memcpy(c, iNode->mainNode->cNode, sizeof(*(iNode->mainNode->cNode)) );
    memcpy(copyCNode, iNode->mainNode->cNode, sizeof(CNode) );

    CNode *updated_cnode = inserted(copyCNode, b, index);  
    std::cout << "temporay CNode created : " << updated_cnode->branch[index]->sNode->value << std::endl;

        if(memcmp(copyCNode, iNode->mainNode->cNode, sizeof(CNode)) == 0){
            std::cout << "mainNode is not changed " << std::endl ;
        }else{
            std::cout << "mainNode is changed" << std::endl;
        }

    bool cas_ret = __sync_bool_compare_and_swap(&iNode->mainNode->cNode, copyCNode, updated_cnode);
    std::cout << "return cas_ret : " << cas_ret << std::endl;
    if(cas_ret){
        std::cout << "New added value " << iNode->mainNode->cNode->branch[index]->sNode->value << std::endl;
        return 0; // successfully attached the node
    }
    else{
        return 1;
    }

The above code is part of my code base. There is no compilation error and the code is running fine. But __sync_bool_compare_and_swap function always return false in my code. Even before the CAS function call I did the memory comparison (memcpy) and its showing that both the arguments are equal, in that case CAS should swap the value with third argument, but it is not.

copyCNode -> Holds the copy value of iNode->mainNode->cNode

updated_cnode -> Holds the updated value of iNode->mainNode->cNode (New branch added)

Please suggest any solution. Thanks

Upvotes: 1

Views: 562

Answers (2)

Django
Django

Reputation: 103

I got the solution.

I think I should not create the memory for copyNode. Instead, I should just copyNode = iNode->mainNode->cNode and the rest code is working good.

Thanks @jxh

Upvotes: 0

jxh
jxh

Reputation: 70402

The CAS operation failed because: iNode->mainNode->cNode ≠ copyCNode. It doesn't matter that their contents are both the same. The CAS operation is trying to do:

if (iNode->mainNode->cNode == copyCNode) {
    iNode->mainNode->cNode = updated_cnode;
    return true;
}
return false;

All you verified is that *copyCnode = *iNode->mainNode->cNode, which is not what the CAS operation is interested in.

Upvotes: 1

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