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

[Jann Horn <jannh@google.com>]

On Wed, May 29, 2019 at 6:07 PM David Howells <dhowells@redhat.com> wrote:
> Greg KH <gregkh@linuxfoundation.org> wrote:
> > everyone should use
> > it.  It saves us having to audit the same pattern over and over again.
> > And, even nicer, it uses a refcount now, and as you are trying to
> > reference count an object, it is exactly what this was written for.
> >
> > So yes, I do think it should be used here, unless it is deemed to not
> > fit the pattern/usage model.
>
> kref_put() enforces a very specific destructor signature.  I know of places
> where that doesn't work because the destructor takes more than one argument
> (granted that this is not the case here).  So why does kref_put() exist at
> all?  Why not kref_dec_and_test()?
>
> Why doesn't refcount_t get merged into kref, or vice versa?  Having both would
> seem redundant.
>
> Mind you, I've been gradually reverting atomic_t-to-refcount_t conversions
> because it seems I'm not allowed refcount_inc/dec_return() and I want to get
> at the point refcount for tracing purposes.

Yeeech, that's horrible, please don't do that.

Does this mean that refcount_read() isn't sufficient for what you want
to do with tracing (because for some reason you actually need to know
the values atomically at the time of increment/decrement)?