linux-kernel - Re: [PATCH printk v4 14/15] printk: extend console

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Message-ID: <87wnfiyv1z.fsf@jogness.linutronix.de>
Date:   Thu, 21 Apr 2022 23:46:24 +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>
Subject: Re: [PATCH printk v4 14/15] printk: extend console_lock for proper
 kthread support

Hi Petr,

If v4 ends up being acceptable for linux-next, I would request you fold
a couple cosmetic changes into this patch.

On 2022-04-21, John Ogness <john.ogness@...utronix.de> wrote:
> +/*
> + * Since the kthread printers do not acquire the console_lock but do need to
> + * access @flags, they could experience races because other tasks
> + * (synchronizing using the console_lock) can modify @flags. These macros are
> + * available to at least provide atomic variable updates so that the kthread
> + * printers can see consistent values.

This last sentence is bad. It should not use the words "atomic" and
"updates". Please change it to:

    These macros are available to store the new value in a way that will
    provide consistent load values for kthread printers. Tasks using
    these macros must still do so under the console_lock.

[...]

>  EXPORT_SYMBOL(console_stop);
>  
> +

Please remove this accidental blank line.

>  void console_start(struct console *console)
>  {
>  	console_lock();
> -	console->flags |= CON_ENABLED;
> -	console_unlock();
>  
> -	/* Wake the newly enabled kthread printer. */
> -	wake_up_klogd();
> +	/* Can cause races for printk_kthread_func(). */
> +	console_flags_set(console->flags, CON_ENABLED);
>  
> +	console_unlock();
>  	__pr_flush(console, 1000, true);
>  }
>  EXPORT_SYMBOL(console_start);

John