The g_wither_geom function tells GEOM that geom gp is to be destroyed. GEOM sets an error on each provider of the given geom (in the orphaning process) and waits for a chance to destroy the geom. If the access count of any possessed consumer goes to 0, the consumer will be detached and destroyed automatically. If the last consumer attached to any possessed provider will be detached, the provider will be destroyed. If there are no more providers nor consumers, the geom will be destroyed.
This is an automatic "garbage collect" to avoid duplicated code in all classes. Before it is called, field softc should be disposed off and set to NULL. Note that the g_wither_geom function gives no guarantee that the geom will be immediately destroyed, mostly because the access counts of the geoms consumers and providers may not be 0. That is why calling this function for every geom from a given class is not enough to be sure that the class can be unloaded.
The argument error must be nonzero.
The topology lock has to be held.
geom(4), DECLARE_GEOM_CLASS(9), g_access(9), g_attach(9), g_bio(9), g_consumer(9), g_data(9), g_event(9), g_geom(9), g_provider(9), g_provider_by_name(9)