Function unp_pcb_lock_peer

freebsd/kern/uipc_usrreq.c:378–419

* Try to lock the connected peer of an already locked socket. In some cases * this requires that we unlock the current socket. The pairbusy counter is * used to block concurrent connection attempts while the lock is dropped. The * caller must be careful to revalidate PCB state. */

 * caller must be careful to revalidate PCB state.
 */
static struct unpcb *
unp_pcb_lock_peer(struct unpcb *unp)
{
	struct unpcb *unp2;

	UNP_PCB_LOCK_ASSERT(unp);
	unp2 = unp->unp_conn;
	if (unp2 == NULL)
		return (NULL);
	if (__predict_false(unp == unp2))
		return (unp);

	UNP_PCB_UNLOCK_ASSERT(unp2);

	if (__predict_true(UNP_PCB_TRYLOCK(unp2)))
		return (unp2);
	if ((uintptr_t)unp2 > (uintptr_t)unp) {
		UNP_PCB_LOCK(unp2);
		return (unp2);
	}
	unp->unp_pairbusy++;
	unp_pcb_hold(unp2);
	UNP_PCB_UNLOCK(unp);

	UNP_PCB_LOCK(unp2);
	UNP_PCB_LOCK(unp);
	KASSERT(unp->unp_conn == unp2 || unp->unp_conn == NULL,
	    ("%s: socket %p was reconnected", __func__, unp));
	if (--unp->unp_pairbusy == 0 && (unp->unp_flags & UNP_WAITING) != 0) {
		unp->unp_flags &= ~UNP_WAITING;
		wakeup(unp);
	}
	if (unp_pcb_rele(unp2)) {
		/* unp2 is unlocked. */
		return (NULL);
	}
	if (unp->unp_conn == NULL) {
		UNP_PCB_UNLOCK(unp2);
		return (NULL);
	}
	return (unp2);
}

/*
 * Definitions of protocols supported in the LOCAL domain.