Using atomics for thread synchronization in C++

In my previous blog post I wrote about spin locks, and how compilers must not move the locking loop above a prior unlock.

After thinking about this done more, I realised that is not something specific to locks — the same issue arises with any two step synchronization between threads.

Consider the following code

std::atomic<bool> ready1{false};
std::atomic<bool> ready2{false};

void thread1(){
  ready1.store(true, std::memory_order_release);
  while(!ready2.load(std::memory_order_acquire)){}
}

void thread2() {
  while(!ready1.load(std::memory_order_acquire)) {}
  ready2.store(true, std::memory_order_release);
}

thread1 sets ready1 to true, then waits for thread2 to set ready2 to true. Meanwhile, thread2 waits for ready1 to be true, then sets ready2 to true.

This is almost identical to the unlock/lock case from the previous blog post, except the waiting thread is just using plain load rather than exchange.

If the compiler moves the wait loop in thread1 above the store then both threads will hang forever. However it cannot do this for the same reason the spinlocks can't deadlock in the previous post: the store has to be visible to the other thread in a finite period if time, so must be issued before the wait loop. https://eel.is/c++draft/intro.multithread#intro.progress-18

An implementation should ensure that the last value (in modification order) assigned by an atomic or synchronization operation will become visible to all other threads in a finite period of time.

If the optimizer moved the store across the loop in thread1, then it could not guarantee that the value became visible to the other thread in a finite period of time. Therefore such an optimization is forbidden.

Posted by Anthony Williams
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