shithub: mc

ref: 3d71446ecb6569add0d9eaf2e7e4a05d99bd2625
dir: /lib/thread/mutex+openbsd:6.2.myr/

use std
use sys

use "atomic"
use "common"

pkg thread =
	type mutex = struct
		_state	: uint32
	;;	

	const mkmtx	: (-> mutex)
	const mtxlock	: (mtx : mutex# -> void)
	const mtxtrylock	: (mtx : mutex# -> bool)
	const mtxunlock	: (mtx : mutex# -> void)

	pkglocal const Unlocked : uint32 = 0
	pkglocal const Locked 	: uint32 = 1
	pkglocal const Contended : uint32 = 2
;;

var nspin = 10	/* FIXME: pick a sane number, based on CPU count */

const mkmtx = {
	-> [._state = Unlocked]
}

const mtxlock = {mtx
	var c

	/* 
	Uncontended case: we get an unlocked mutex, and we lock it.
	*/
        c = Locked
	for var i = 0; i < nspin; i++
		c = xcas(&mtx._state, Unlocked, Locked) 
		if c == Unlocked
			-> void
		;;
	;;

	/*
	Contended case: we set the lock state to Contended. This indicates that there
	the lock is locked, and we potentially have threads waiting on it, which means
	that we will need to wake them up.
	*/
	if c == Locked
		c = xchg(&mtx._state, Contended)
	;;

	while c != Unlocked
		sys.futex(&mtx._state, sys.Futexwait, (Contended : int), Zptr, Zptr)
		c = xchg(&mtx._state, Contended)
	;;
}

const mtxtrylock = {mtx
	-> xcas(&mtx._state, Unlocked, Locked) == Unlocked
}

const mtxunlock = {mtx
	/*
	Uncontended case: If the mutex state is not contended, and we still
	are uncontended by the xchg() call, then it's safe to simply return;
	nobody was waiting for us.
	*/
	if mtx._state == Contended
		mtx._state = Unlocked
	elif xchg(&mtx._state, Unlocked) == Locked
		-> void
	;;

	/* wake one thread */
	sys.futex(&mtx._state, sys.Futexwake, 1, Zptr, Zptr)
}