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From:   Eric Dumazet <edumazet@google.com>
Date:   Wed, 7 Sep 2022 15:58:12 -0700
Message-ID: <CANn89iLXsSotHWkUv4h0jCyNqym+Mb1N2-sfyC0sK76TZ+xPPg@mail.gmail.com>
Subject: Re: [PATCH v8 08/26] tcp: authopt: Disable via sysctl by default
To:     Herbert Xu <herbert@gondor.apana.org.au>
Cc:     Leonard Crestez <cdleonard@gmail.com>,
        David Ahern <dsahern@kernel.org>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        Francesco Ruggeri <fruggeri@arista.com>,
        Salam Noureddine <noureddine@arista.com>,
        Philip Paeps <philip@trouble.is>,
        Shuah Khan <shuah@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Kuniyuki Iwashima <kuniyu@amazon.co.jp>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Jakub Kicinski <kuba@kernel.org>,
        Yuchung Cheng <ycheng@google.com>,
        Mat Martineau <mathew.j.martineau@linux.intel.com>,
        Christoph Paasch <cpaasch@apple.com>,
        Ivan Delalande <colona@arista.com>,
        Caowangbao <caowangbao@huawei.com>,
        Priyaranjan Jha <priyarjha@google.com>,
        netdev <netdev@vger.kernel.org>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@vger.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, Sep 7, 2022 at 3:50 PM Herbert Xu <herbert@gondor.apana.org.au> wrote:
>
> On Tue, Sep 06, 2022 at 04:11:58PM -0700, Eric Dumazet wrote:
> >
> > WRITE_ONCE(sysctl_tcp_authopt, val),  or even better:
> >
> > if (val)
> >      cmpxchg(&sysctl_tcp_authopt, 0, val);
>
> What's the point of the cmpxchg? Since you're simply trying to prevent
> sysctl_tcp_authopt from going back to zero, then the if clause
> by itself is enough:
>
>         if (val)
>                 WRITE_ONCE(sysctl_tcp_authopt, val);
>

Ack.

Original patch was doing something racy, I have not though about the
most efficient way to deal with it.