A buffered device block whose identity is retained for unfinished work. Reference counting can represent both client references and additional pins. Dropping a client reference does not release an outstanding journal pin; dropping a pin does not remove somebody else's reference. In the buffer cache state model, counts all such retained references, so prevents buffer cache eviction. This count keeps the structure alive; a mutex separately controls who can inspect or change its contents. Waiting callers also need lifetime protection.
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