Re: [PATCH 1/7] General notification queue with user mmap()'able ring buffer

Peter Zijlstra
2019-05-31T16:45:08Z

On Fri, May 31, 2019 at 03:20:12PM +0100, David Howells wrote:
> Peter Zijlstra <peterz@infradead.org> wrote:

> > (and it has already been established that refcount_t doesn't work for
> > usage count scenarios)
>
> ?
>
> Does that mean struct kref doesn't either?

Indeed, since kref is just a pointless wrapper around refcount_t it does
not either.

The main distinction between a reference count and a usage count is that
0 means different things. For a refcount 0 means dead. For a usage count
0 is merely unused but valid.

Incrementing a 0 refcount is a serious bug -- use-after-free (and hence
refcount_t will refuse this and splat), for a usage count this is no
problem.

Now, it is sort-of possible to merge the two, by basically stating
something like: usage = refcount - 1. But that can get tricky and people
have not really liked the result much for the few times I tried.