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

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Message-ID: <87cz4tus9d.fsf@jogness.linutronix.de>
Date:   Tue, 28 Mar 2023 11:48:06 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
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 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).

John