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

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Date:   Wed, 29 May 2019 19:46:54 +0200
From:   Jann Horn <jannh@...gle.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Al Viro <viro@...iv.linux.org.uk>, raven@...maw.net,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-block@...r.kernel.org, keyrings@...r.kernel.org,
        linux-security-module <linux-security-module@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 1/7] General notification queue with user mmap()'able ring buffer

On Wed, May 29, 2019 at 6:07 PM David Howells <dhowells@...hat.com> wrote:
> Greg KH <gregkh@...uxfoundation.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)?