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

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Date:   Thu, 16 Mar 2017 09:58:54 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Elena Reshetova <elena.reshetova@...el.com>
Cc:     netdev@...r.kernel.org, bridge@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, kuznet@....inr.ac.ru,
        jmorris@...ei.org, kaber@...sh.net, stephen@...workplumber.org,
        peterz@...radead.org, keescook@...omium.org,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Windsor <dwindsor@...il.com>
Subject: Re: [PATCH 07/17] net: convert sock.sk_refcnt from atomic_t to
 refcount_t

On Thu, 2017-03-16 at 17:28 +0200, Elena Reshetova wrote:
> refcount_t type and corresponding API should be
> used instead of atomic_t when the variable is used as
> a reference counter. This allows to avoid accidental
> refcounter overflows that might lead to use-after-free
> situations.

...

>  static __always_inline void sock_hold(struct sock *sk)
>  {
> -	atomic_inc(&sk->sk_refcnt);
> +	refcount_inc(&sk->sk_refcnt);
>  }
>  

While I certainly see the value of these refcount_t, we have a very
different behavior on these atomic_inc() which were doing a single
inlined LOCK RMW on x86.

We now call an external function performing a 
atomic_read(), various ops/tests, then atomic_cmpxchg_relaxed(), in a
loop, loosing the nice ability for x86 of preventing live locks.

Looks a lot of bloat, just to be able to chase hypothetical bugs in the
kernel.

I would love to have a way to enable extra debugging when I want a debug
kernel, like LOCKDEP or KASAN.

By adding all this bloat, we assert linux kernel is terminally buggy and
every atomic_inc() we did was suspicious, and need to be always
instrumented/validated.