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From: David Windsor <dwindsor@gmail.com>
Date: Mon, 21 Nov 2016 15:12:33 -0500
Message-ID: <CAEXv5_hXEOkwQw6zaood7f4Pugqn6ELsa4w89eD2Umv5m5Pyug@mail.gmail.com>
Subject: Re: [RFC][PATCH 2/7] kref: Add kref_read()
To: "Reshetova, Elena" <elena.reshetova@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Will Deacon <will.deacon@arm.com>, Arnd Bergmann <arnd@arndb.de>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, LKML <linux-kernel@vger.kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>
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Sender: linux-kernel-owner@vger.kernel.org
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On Mon, Nov 21, 2016 at 2:27 PM, Reshetova, Elena
<elena.reshetova@intel.com> wrote:
>> 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?

Many of the calls to non-supported functions can be decomposed into
calls to supported functions.  The ones that may prove interesting are
ones like atomic_cmpxchg(), in which some sort of external locking is
going to be required to achieve the same atomicity guarantees provided
by cmpxchg, like so:

mutex_lock(lock);
cnt = refcount_read(ref);
if (cnt == val1) {
    refcount_set(ref, val2);
}
mutex_unlock(lock);
return cnt;