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

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Message-ID: <20170320142807.mhu3xod2x2zzvego@hirez.programming.kicks-ass.net>
Date:   Mon, 20 Mar 2017 15:28:07 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Herbert Xu' <herbert@...dor.apana.org.au>,
        David Miller <davem@...emloft.net>,
        "eric.dumazet@...il.com" <eric.dumazet@...il.com>,
        "elena.reshetova@...el.com" <elena.reshetova@...el.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bridge@...ts.linux-foundation.org" 
        <bridge@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kuznet@....inr.ac.ru" <kuznet@....inr.ac.ru>,
        "jmorris@...ei.org" <jmorris@...ei.org>,
        "kaber@...sh.net" <kaber@...sh.net>,
        "stephen@...workplumber.org" <stephen@...workplumber.org>,
        "ishkamiel@...il.com" <ishkamiel@...il.com>,
        "dwindsor@...il.com" <dwindsor@...il.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH 07/17] net: convert sock.sk_refcnt from atomic_t to
 refcount_t

On Mon, Mar 20, 2017 at 02:10:24PM +0000, David Laight wrote:
> On x86 the cpu flags from the 'lock inc/dec' could be used to reasonably
> cheaply detect errors - provided you actually generate a forwards branch.

Note that currently there is no arch specific implementation. We could
of course cure this.

But note that the thing you propose; using the overflow flag, can only
reasonably be done on PREEMPT=n kernels, otherwise we have an incredible
number of contexts that can nest.

Sure; getting all starts aligned to double overflow is incredibly rare,
but I don't want to be the one to have to debug that.