Is "atomic_mutex_try_lock" what I think it is?

Explain
1

My understanding is that an atomic_mutex spins for a bit and then sleeps using a futex if it can’t get the lock. If it gets the lock then performance is identical to a spinlock.

Then is atomic_mutex_try_lock something that behaves like a compare_exchange which is lock free?

I’m trying to decide if I should just use this instead of using atomics directly for a job system. If there are performance reasons not to use it then I’d like to know them.

EDIT: Okay, this is going to sound dumb. I assumed since other sync primitives didn’t have source code when I checked then this wouldn’t have it either. I just checked again and it’s there clear as day. It does indeed behave how I think it does, sorry for the pointless post.

EDIT2: I guess I can ask something else I’m curious about. Why use a mutex when atomic_mutex exists?

2

Atomic_Mutex is the native Odin implementation of a mutex. Mutex is an abstraction layer that encompasses Atomic_Mutex on all non-Windows platforms. On Windows, we use shared reader-writer locks which are more performant. Use Mutex.

The various try_lock procedures will not block if you run them, so yes, they are lock-free.

The code is in fact just a compare_exchange as you can see:

// atomic_mutex_try_lock tries to lock m, will return true on success, and false on failure
atomic_mutex_try_lock :: proc "contextless" (m: ^Atomic_Mutex) -> bool {
	_, ok := atomic_compare_exchange_strong_explicit(&m.state, .Unlocked, .Locked, .Acquire, .Consume)
	return ok
}

I’m not sure what this means. You should have source code for everything in core.

1 Like
3

Blockquote
I’m not sure what this means. You should have source code for everything in core.

I mean like the mutex for example. Is there source for the _Mutex type somewhere? Thanks for answering by the way.

Mutex :: struct #no_copy {
	impl: _Mutex,
}

mutex_unlock :: proc "contextless" (m: ^Mutex) {
	_mutex_unlock(m)
}

mutex_try_lock :: proc "contextless" (m: ^Mutex) -> bool {
	return _mutex_try_lock(m)
}
4

Indeed. I just grepped for _Mutex :: and found the non-Windows implementation under primitives_internal.odin and the Windows implementation under primitives_windows.odin.

1 Like
5

I’ll go check those out. Sorry for the trouble.