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

The guarantee of a std::mutex is enable exclusive access to shared resources. Its sole purpose is to eliminate the race condition when multiple threads attempt to access shared resources.

The implementer of a mutex may choose to favor the current thread acquiring a mutex (over another thread) for performance reasons. Allowing the current thread to acquire the mutex and make forward progress without requiring a context switch is often a preferred implementation choice supported by profiling/measurements.

Alternatively, the mutex could be constructed to prefer another (blocked) thread for acquisition (perhaps chosen according FIFO). This likely requires a thread context switch (on the same or other processor core) increasing latency/overhead. NOTE: FIFO mutexes can behave in surprising ways. E.g. Thread priorities must be considered in FIFO support - so acquisition won't be strictly FIFO unless all competing threads are the same priority.

Adding a FIFO requirement to a mutex's definition constrains implementers to provide suboptimal performance in nominal workloads. (see above)

Protecting a queue of callable objects (std::function) with a mutex would enable sequenced execution. Multiple threads can acquire the mutex, enqueue a callable object, and release the mutex. The callable objects can be executed by a single thread (or a pool of threads if synchrony is not required).