15.2.6.5 Atomic update of a data structure

The example below is a resource object, which maintains a count of free items and also list of them. These two slots must stay synchronized.

Old:

(without-interrupts

 (when (plusp (resource-free-item-count resource))

   (decf (resource-free-item-count resource))

   (pop (resource-free-items resource))))

New: use a lock, because more than one slot has to be updated, so cannot be updated with low level atomic operations.

(mp:with-lock ((resource-lock resource))

	      (when (plusp (resource-free-item-count resource))

		(decf (resource-free-item-count resource))

		(pop (resource-free-items resource))))