linux-kernel - Re: [PATCH] once: fix race by moving DO

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Message-ID: <CANn89iLi4CQZhAw7DKVauk0+cC+nBjoVuHgAan=cOsCP07Jh=w@mail.gmail.com>
Date: Sat, 6 Sep 2025 07:34:19 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Qi Xi <xiqi2@...wei.com>
Cc: bobo.shaobowang@...wei.com, xiexiuqi@...wei.com, arnd@...db.de, 
	masahiroy@...nel.org, kuba@...nel.org, linux-arch@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] once: fix race by moving DO_ONCE to separate section

On Sat, Sep 6, 2025 at 6:58 AM Qi Xi <xiqi2@...wei.com> wrote:
>
> The commit c2c60ea37e5b ("once: use __section(".data.once")") moved
> DO_ONCE's ___done variable to .data.once section, which conflicts with
> WARN_ONCE series macros that also use the same section.
>
> This creates a race condition when clear_warn_once is used:
>
> Thread 1 (DO_ONCE)             Thread 2 (DO_ONCE)
> __do_once_start
>     read ___done (false)
>     acquire once_lock
> execute func
> __do_once_done
>     write ___done (true)      __do_once_start
>     release once_lock             // Thread 3 clear_warn_once reset ___done
>                                   read ___done (false)
>                                   acquire once_lock
>                               execute func
> schedule once_work            __do_once_done
> once_deferred: OK                 write ___done (true)
> static_branch_disable             release once_lock
>                               schedule once_work
>                               once_deferred:
>                                   BUG_ON(!static_key_enabled)

Should we  use this section as well in include/linux/once_lite.h ?

Or add a comment there explaining that there is a difference
between the two variants, I am not sure this was explicitly mentioned
in the past.