Function lf_cancel_lock

freebsd/kern/kern_lockf.c:1326–1372

* Cancel a pending lock request, either as a result of a signal or a * cancel request for an async lock. */

 * cancel request for an async lock.
 */
static void
lf_cancel_lock(struct lockf *state, struct lockf_entry *lock)
{
	struct lockf_entry_list granted;

	/*
	 * Note it is theoretically possible that cancelling this lock
	 * may allow some other pending lock to become
	 * active. Consider this case:
	 *
	 * Owner	Action		Result		Dependencies
	 * 
	 * A:		lock [0..0]	succeeds	
	 * B:		lock [2..2]	succeeds	
	 * C:		lock [1..2]	blocked		C->B
	 * D:		lock [0..1]	blocked		C->B,D->A,D->C
	 * A:		unlock [0..0]			C->B,D->C
	 * C:		cancel [1..2]	
	 */

	LIST_REMOVE(lock, lf_link);

	/*
	 * Removing out-going edges is simple.
	 */
	sx_xlock(&lf_owner_graph_lock);
	lf_remove_outgoing(lock);
	sx_xunlock(&lf_owner_graph_lock);

	/*
	 * Removing in-coming edges may allow some other lock to
	 * become active - we use lf_update_dependancies to figure
	 * this out.
	 */
	LIST_INIT(&granted);
	lf_update_dependancies(state, lock, TRUE, &granted);
	lf_free_lock(lock);

	/*
	 * Feed any newly active locks to lf_activate_lock.
	 */
	while (!LIST_EMPTY(&granted)) {
		lock = LIST_FIRST(&granted);
		LIST_REMOVE(lock, lf_link);
		lf_activate_lock(state, lock);
	}
}

/*
 * Set a byte-range lock.