On Mon, May 29, 2017 at 06:39:44AM -0500, Eric W. Biederman wrote: > I failed to see that there is a refcount_inc. Too much noise in > the header file I suppose. > > But implementing refcount_inc in terms of refcount_inc_not_zero is > totally broken. The two operations are not the same and the go to > different assumptions the code is making. > > That explains why you think refcount_inc_not_zero should lie because > you are implementing refcount_inc with it. They are semantically very > different operations. Please separate them. There has been much debate about this. And the best I'll do is add a comment and/or retain these exact semantics. What is done is: refcount_inc() := WARN_ON(!refcount_inc_not_zero()) Because incrementing a zero reference count is a use-after-free and something we should not do ever. This is where the whole usage count vs reference count pain comes from. Once there are no more _references_ to an object, a reference count frees the object. Therefore a zero reference count means a dead object and incrementing from that is fail. The usage count model otoh counts how many (active) users there are of an object, and no active users is a good and expected situation. But it is very explicitly not a reference count. Because even in the no users case do we have a reference to the object (we've not leaked it after all, we just don't track all references). Similarly, refcount_dec() is implemented using dec_and_test() and will WARN when it hits 0, because this is a leak and we don't want those either. A usage count variant otoh would be fine with hitting 0.
