netdev - Re: [PATCH v1 net 03/16] sysctl: Add proc_dointvec

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Date:   Wed, 6 Jul 2022 09:00:11 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Kuniyuki Iwashima <kuniyu@...zon.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        Kuniyuki Iwashima <kuni1840@...il.com>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 net 03/16] sysctl: Add proc_dointvec_lockless().

On Wed, Jul 6, 2022 at 7:22 AM Kuniyuki Iwashima <kuniyu@...zon.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.