shithub: mc

ref: b5ddbc86346fddc60360a817ee62bc8ca938973e
dir: /lib/thread/sem+futex.myr/

use std

use "atomic"
use "futex"

pkg thread =
	/*
	We want to be able to read both members in the same atomic operation
	and this implementation assumes `ftxtag` is 32 bits.
	*/
	type sem = struct
		_val      : ftxtag
		_nwaiters : uint32
	;;

	const mksem      : (v : uint32 -> sem)
	const semwait    : (s : sem# -> void)
	const semtrywait : (s : sem# -> bool)
	const sempost    : (s : sem# -> void)
;;

const mksem = {v
	-> [._val = (v : ftxtag)]
}

const semwait = {s
	var v = 0

	xadd(&s._nwaiters, 1)
	for ; ;
		while (v = s._val) > 0
			if xcas(&s._val, v, v - 1) == v
				xadd(&s._nwaiters, -1)
				-> void
			;;
		;;
		ftxwait(&s._val, v, -1)
	;;
}

const semtrywait = {s
	for ; ;
		var v = xget(&s._val)
		if v == 0
			-> false
		;;
		if xcas(&s._val, v, v - 1) == v
			-> true
		;;
	;;
	-> false /* Unreachable */
}

const sempost = {s
	/*
	It's possible for the semaphore to be deallocated at any time after
	`_val` is incremented so we must not dereference `s` a second time. We
	work around this by also reading the value of `_nwaiters` during the
	atomic fetch and add.
	*/
	var state = xadd((s : uint64#), 1)
	std.assert((state : uint32) != ~0x0, "error: semaphore overflowed\n")

	if (state >> 32) > 0
		/*
		However, it is both legal and expected to pass a potentially
		invalid address to `ftxwake`.
		*/
		ftxwake(&s._val)
	;;
}