Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20220706052130.16368-1-kuniyu@amazon.com> <20220706052130.16368-4-kuniyu@amazon.com>
In-Reply-To: <20220706052130.16368-4-kuniyu@amazon.com>
From:   Eric Dumazet <edumazet@google.com>
Date:   Wed, 6 Jul 2022 09:00:11 +0200
Message-ID: <CANn89i+mJCN=4h5-MM5jUQN8Hv=NdyTmQQb7Oeop+DyYVcEWUg@mail.gmail.com>
Subject: Re: [PATCH v1 net 03/16] sysctl: Add proc_dointvec_lockless().
To:     Kuniyuki Iwashima <kuniyu@amazon.com>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>,
        Kuniyuki Iwashima <kuni1840@gmail.com>,
        netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Wed, Jul 6, 2022 at 7:22 AM Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
>
> A sysctl variable is accessed concurrently, and there is always a chance of
> data-race.  So, all readers and writers need some basic protection to avoid
> load/store-tearing.
>
> This patch changes proc_dointvec() to use READ_ONCE()/WRITE_ONCE()
> internally to fix a data-race on the sysctl side.  For now, proc_dointvec()
> itself is tolerant to a data-race, but we still need to add annotations on
> the other subsystem's side.
>
> In case we miss such fixes, this patch converts proc_dointvec() to a
> wrapper of proc_dointvec_lockless().  When we fix a data-race in the other
> subsystem, we can explicitly set it as a handler.
>
> Also, this patch removes proc_dointvec()'s document and adds
> proc_dointvec_lockless()'s one so that no one will use proc_dointvec()
> anymore.
>
> While we are on it, we remove some trailing spaces.


I do not see why you add more functions.

Really all sysctls can change locklessly by nature, as I pointed out.

So I would simply add WRITE_ONCE() whenever they are written, and
READ_ONCE() when they are read.

If stable teams care enough, they will have to backport these changes,
so I would rather not have to change
proc_dointvec() to proc_dointvec_lockless() in many files, with many
conflicts, that ultimately will either
add bugs, or ask extra work for maintainers.