shithub: mc

ref: 23156b142baa61ea67cec46d7f4da141c19f91bd
dir: /lib/thread/condvar+freebsd.myr/

use std

use "atomic"
use "mutex"
use "futex"

pkg thread =
	type cond = struct
		_mtx	: mutex#
		_seq	: ftxtag
	;;

	const mkcond	: (mtx : mutex# -> cond)
	const condwait	: (cond : cond# -> void)
	const condsignal	: (cond : cond# -> void)
	const condbroadcast	: (cond : cond# -> void)
;;

const mkcond = {mtx
	-> [._mtx = mtx, ._seq = 0]
}

const condwait = {cond
	var mtx = cond._mtx
	var seq = xget(&cond._seq)

	mtxunlock(mtx)
	ftxwait(&cond._seq, seq, -1)

	/*
	In the event of a broadcast, we aren't requeueing waiters from the
	condvar's futex to the mutex's futex so we can just reacquire the mutex
	normally.
	*/
	mtxlock(mtx)
}

const condsignal = {cond : cond#
	xadd(&cond._seq, 1)
	ftxwake(&cond._seq)
}

/*
`umtx_op` fully supports implementing condvars efficiently but also requires
condvars to be implemented in a specific way. For now we'll just invite the
thundering herd.
*/
const condbroadcast = {cond : cond#
	xadd(&cond._seq, 1)
	ftxwakeall(&cond._seq)
}