linux-kernel - RE: [RFC][PATCH 2/7] kref: Add kref

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Date:   Mon, 21 Nov 2016 19:27:29 +0000
From:   "Reshetova, Elena" <elena.reshetova@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     David Windsor <dwindsor@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        "Arnd Bergmann" <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: RE: [RFC][PATCH 2/7] kref: Add kref_read()

> On Mon, Nov 21, 2016 at 04:49:15PM +0100, Peter Zijlstra wrote:
> > > Speaking of non-fitting patterns. This one is quite common in
> > > networking code for refcounters:
> > >
> > > if (atomic_cmpxchg(&cur->refcnt, 1, 0) == 1) {} This is from
> > > net/netfilter/nfnetlink_acct.c, but there are similar ones in other
> > > places.
> >
> > Cute, but weird it doesn't actually decrement if not 1.
> 
> Hurgh.. creative refcounting that. The question is how much of that do
> we want to support? It really must not decrement there.

And one more creative usage:

http://lxr.free-electrons.com/source/net/ipv4/udp.c#L1940

if (!sk || !atomic_inc_not_zero_hint(&sk->sk_refcnt, 2))
    return;

I didn't even guess anyone is using atomic_inc_not_zero_hint... 
But network code keeps surprising me today :)
So, yes, I guess the question is what to do with these cases really?