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It’s atomic even though the CAS loop may take an indeterminate number of tries, because when the loop finally does modify shared, it does so atomically. The above implementation of fetch_multiply is both atomic and lock-free. In that case, oldValue will be updated with the most recent value of shared, and the loop will try again. On the other hand, if shared is concurrently modified by another thread, it’s totally possible for its value to change between the call to load and the call to compare_exchange_weak, causing the compare-and-swap operation to fail. If no concurrent modifications happen in other threads, compare_exchange_weak will usually succeed on the first try. The function repeatedly tries to exchange oldValue with oldValue * multiplier until it succeeds. This is known as a compare-and-swap loop, or CAS loop. compare_exchange_weak(oldValue, oldValue * multiplier)) Uint32_t fetch_multiply(std::atomic& shared, uint32_t multiplier) Every other RMW operation can be implemented using that one. Out of all the available RMW operations in C++11, the only one that is absolutely essential is compare_exchange_weak. As the title says, you can do any kind of RMW operation! Because if you do need those operations, you can easily implement them yourself.
INTEL C COMPILER ATOMIC OPERATIONS CODE
You can’t write safe multithreaded code by taking a single-threaded algorithm and turning each step into an RMW. Try not to get the wrong impression of how RMWs are used.
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