Function unp_gc

freebsd/kern/uipc_usrreq.c:2649–2777

    "Number of active local sockets.");

static void
unp_gc(__unused void *arg, int pending)
{
	struct unp_head *heads[] = { &unp_dhead, &unp_shead, &unp_sphead,
				    NULL };
	struct unp_head **head;
	struct unp_head unp_deadhead;	/* List of potentially-dead sockets. */
	struct file *f, **unref;
	struct unpcb *unp, *unptmp;
	int i, total, unp_unreachable;

	LIST_INIT(&unp_deadhead);
	unp_taskcount++;
	UNP_LINK_RLOCK();
	/*
	 * First determine which sockets may be in cycles.
	 */
	unp_unreachable = 0;

	for (head = heads; *head != NULL; head++)
		LIST_FOREACH(unp, *head, unp_link) {
			KASSERT((unp->unp_gcflag & ~UNPGC_IGNORE_RIGHTS) == 0,
			    ("%s: unp %p has unexpected gc flags 0x%x",
			    __func__, unp, (unsigned int)unp->unp_gcflag));

			f = unp->unp_file;

			/*
			 * Check for an unreachable socket potentially in a
			 * cycle.  It must be in a queue as indicated by
			 * msgcount, and this must equal the file reference
			 * count.  Note that when msgcount is 0 the file is
			 * NULL.
			 */
			if (f != NULL && unp->unp_msgcount != 0 &&
			    refcount_load(&f->f_count) == unp->unp_msgcount) {
				LIST_INSERT_HEAD(&unp_deadhead, unp, unp_dead);
				unp->unp_gcflag |= UNPGC_DEAD;
				unp->unp_gcrefs = unp->unp_msgcount;
				unp_unreachable++;
			}
		}

	/*
	 * Scan all sockets previously marked as potentially being in a cycle
	 * and remove the references each socket holds on any UNPGC_DEAD
	 * sockets in its queue.  After this step, all remaining references on
	 * sockets marked UNPGC_DEAD should not be part of any cycle.
	 */
	LIST_FOREACH(unp, &unp_deadhead, unp_dead)
		unp_gc_scan(unp, unp_remove_dead_ref);

	/*
	 * If a socket still has a non-negative refcount, it cannot be in a
	 * cycle.  In this case increment refcount of all children iteratively.
	 * Stop the scan once we do a complete loop without discovering
	 * a new reachable socket.
	 */