shithub: mc

ref: a4eb9c0aca8ae4adb6c50133ba26bbd250ae934d
dir: /lib/thread/rwlock.myr/

use "mutex"

pkg thread =
	/*
	`_lock` grants exclusive access to either n readers or one writer.

	`_rlock` protects `_rcount` and allows the first reader to attempt to
	lock `_lockk` without letting other readers in.
	*/
	type rwlock = struct
		_lock   : mutex
		_rlock  : mutex
		_rcount : uint32
	;;

	const mkrwlock  : (-> rwlock)
	const rdlock    : (rw : rwlock# -> void)
	const wrlock    : (rw : rwlock# -> void)
	const tryrdlock : (rw : rwlock# -> bool)
	const trywrlock : (rw : rwlock# -> bool)
	const rdunlock  : (rw : rwlock# -> void)
	const wrunlock  : (rw : rwlock# -> void)
;;

const mkrwlock = {
	-> [._lock = mkmtx(), ._rlock = mkmtx()]
}

const rdlock = {rw
	mtxlock(&rw._rlock)

	/*
	The first reader either successfully acquires `_lock`, locking out all
	writers, or blocks while holding `_rlock`, locking out all other
	readers until it is able to acquire `_lock`, meaning that a writer has
	released it.
	*/
	if rw._rcount++ == 0
		mtxlock(&rw._lock)
	;;
	mtxunlock(&rw._rlock)
}

const wrlock = {rw
	mtxlock(&rw._lock)
}

const tryrdlock = {rw
	var rc = true
	mtxlock(&rw._rlock)
	if rw._rcount++ == 0
		if !mtxtrylock(&rw._lock)
			rw._rcount--
			rc = false
		;;
	;;
	mtxunlock(&rw._rlock)
	-> rc
}

const trywrlock = {rw
	-> mtxtrylock(&rw._lock)
}

const rdunlock = {rw
	mtxlock(&rw._rlock)

	/* `_lock` is not released until the last reader releases the lock. */
	if --rw._rcount == 0
		mtxunlock(&rw._lock)
	;;
	mtxunlock(&rw._rlock)

}

const wrunlock = {rw
	mtxunlock(&rw._lock)
}