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

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Date:   Wed, 22 Mar 2017 13:25:40 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
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 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() ?

That said; I'm really loath to provide these without actual measurements
that prove they make a difference.

> Then later, maybe provide a dynamic infrastructure so that we can
> dynamically force the full checks even for refcount_inc_relaxed() on say
> 1% of the hosts, to get better debug coverage ?

Shouldn't be too hard to do in arch specific code using alternative
stuff. Generic code could use jump labels I suppose, but that would
result in bigger code.