linux-kernel - Re: union: was: Re: [PATCH printk v1 05/18] printk: Add non-BKL console basic infrastructure

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Message-ID: <ZCLi8kWOdq8dwxSH@alley>
Date:   Tue, 28 Mar 2023 14:52:02 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-fsdevel@...r.kernel.org
Subject: Re: union: was: Re: [PATCH printk v1 05/18] printk: Add non-BKL
 console basic infrastructure

On Tue 2023-03-28 11:48:06, John Ogness wrote:
> On 2023-03-28, Petr Mladek <pmladek@...e.com> wrote:
> >> A compilation check would be nice. Is that possible?
> >
> > I think the following might do the trick:
> >
> > static_assert(sizeof(struct cons_state) == sizeof(atomic_long_t));
> 
> I never realized the kernel code was allowed to have that. But it is
> everywhere! :-) Thanks. I've added and tested the following:
> 
> /*
>  * The nbcon_state struct is used to easily create and interpret values that
>  * are stored in the console.nbcon_state variable. Make sure this struct stays
>  * within the size boundaries of that atomic variable's underlying type in
>  * order to avoid any accidental truncation.
>  */
> static_assert(sizeof(struct nbcon_state) <= sizeof(long));
> 
> Note that I am checking against sizeof(long), the underlying variable
> type. We probably shouldn't assume sizeof(atomic_long_t) is always
> sizeof(long).

Makes sense and looks good to me.

Best Regards,
Petr