Do mutexes guarantee ordering of acquisition?

Question

A coworker had an issue recently that boiled down to what we believe was the following sequence of events in a C++ application with two threads:

• Thread A ho

Answer 1

How do you know this:

While thread A is holding the mutex, thread B attempts to lock it. Since it is held, thread B is suspended.

How do you know thread B is suspended. How do you know that it is not just finished the line of code before trying to grab the lock, but not yet grabbed the lock:

Thread B:

x = 17; // is the thread here?
// or here? ('between' lines of code)
mtx.lock();  // or suspended in here?
// how can you tell?

You can't tell. At least not in theory.

Thus the order of acquiring the lock is, to the abstract machine (ie the language), not definable.