netdev - Re: [PATCH 07/17] net: convert sock.sk_refcnt from atomic_t to refcount

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Message-ID: <1490188936.16816.143.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Wed, 22 Mar 2017 06:22:16 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        David Miller <davem@...emloft.net>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Network Development <netdev@...r.kernel.org>,
        bridge@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        James Morris <jmorris@...ei.org>,
        Patrick McHardy <kaber@...sh.net>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Windsor <dwindsor@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 07/17] net: convert sock.sk_refcnt from atomic_t to
 refcount_t

On Wed, 2017-03-22 at 13:25 +0100, Peter Zijlstra wrote:
> On Tue, Mar 21, 2017 at 07:03:19PM -0700, Eric Dumazet wrote:
> 
> > Note that we might define two refcount_inc()  : One that does whole
> > tests, and refcount_inc_relaxed() that might translate to atomic_inc()
> > on non debug kernels.
> 
> So you'd want a duplicate interface, such that most code, which doesn't
> care about refcount performance much, can still have all the tests
> enabled.
> 
> But the code that cares about it (and preferably can prove it with
> numbers) can use the other.
> 
> I'm also somewhat hesitant to use _relaxed for this distinction, as it
> has a clear meaning in atomics, maybe _nocheck?
> 
> Also; what operations do you want _nocheck variants of, only
> refcount_inc() ?

I was mostly thinking of points where we were already checking the value
either before or after the atomic_inc(), using some lazy check (a la
WARN_ON(atomic_read(p) == 0) or something like that.

But admittedly we can replace all these by standard refcount_inc() and
simply provide a CONFIG option to turn off the checks, and let brave
people enable this option.