func addExtraM(mp *m) {
	mnext := lockextra(true)
	mp.schedlink.set(mnext)
	unlockextra(mp, 1)
}

var (
	// allocmLock is locked for read when creating new Ms in allocm and their
	// addition to allm. Thus acquiring this lock for write blocks the
	// creation of new Ms.
	allocmLock rwmutex

	// execLock serializes exec and clone to avoid bugs or unspecified