haiku/docs/develop/kernel/VM_Locking

Locks/Reference Counting:

vm_address_space:
	sem R/W for area creation/deletion and any other address space changes
	fields: areas, area_hint (is currently written in vm_area_lookup() without a write lock!),
		state
	ref_count: ensures validity of object beyond team lifetime,
		retrieved via the global sAddressSpaceTable's pointer (which is guarded by
			sAddressSpaceHashSem)
		vm_address_space_walk_next() is unsafe! (and obsolete, only used by the former
			page scanner)
	Problems: resize_area() does not lock any address spaces yet, but needs to lock all clones

vm_area:
	ref_count: ensures validity
		retrieved via the global sAreaHash's pointer (which is guarded by
			sAreaHashLock)
		vs. vm_area_lookup() which iterates over the address space's area list, not
			the hash - therefore, it checks ref_count against NULL (ugly)
	variable fields:
		size, protection: essentially unguarded! (can be changed by resize_area()
			and set_area_protection())
		mappings: guarded by the global sMappingLock (currently a spinlock)
		address_space_next: vm_address_space::sem
		hash_next: sAreaHashLock
		cache: guarded by vm_area_get_locked_cache()/sAreaCacheLock
		cache_next|prev: cache_ref::lock

vm_cache_ref:
	ref_count: ensures validity
		vm_cache_remove_consumer(): does scary things with the ref_count
		fault_acquire_locked_source(): tries to get a ref through the vm_cache
	cache, areas: guarded by lock

vm_cache:
	all fields: guarded by ref::lock
	BUT: ref may change, therefore it's generally unsafe to go from cache to ref
		without holding the ref's lock (which happens, by design, in vm_cache::source
		and vm_cache::consumers)!

vm_page:
	hash_next: guarded by sPageCacheTableLock spinlock
	queue_prev|next: guarded by sPageLock
	cache_prev|next, cache, cache_offset: guarded by vm_cache_ref::lock
	mappings: guarded by the global sMappingLock (currently a spinlock)
	state: in vm_page only used with the sPageLock held, other uses have the
		cache locked the page is in
	wired_count, usage_count: not guarded? TBD
	busy_reading, busy_writing: dummy pages only

vm_translation_map: 
	TBD.