In /sys/src/9/port/proc.c a comment state:

/*
* Expects that only one process can call wakeup for any given Rendez.
* We hold both locks to ensure that r->p and p->r remain consistent.
* Richard Miller has a better solution that doesn't require both to
* be held simultaneously, but I'm a paranoid - presotto.
*/

(see https://github.com/0intro/plan9/blob/master/sys/src/9/port/proc.c#L882-L887)