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Date:   Wed, 14 Sep 2022 21:31:57 +0900
From:   Sergey Senozhatsky <senozhatsky@chromium.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        John Ogness <john.ogness@linutronix.de>,
        Petr Mladek <pmladek@suse.com>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Daniel Vetter <daniel@ffwll.ch>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Helge Deller <deller@gmx.de>,
        Jason Wessel <jason.wessel@windriver.com>,
        Daniel Thompson <daniel.thompson@linaro.org>
Subject: Re: [patch RFC 06/29] printk: Protect [un]register_console() with a
 mutex
Message-ID: <YyHJvdQbmd1wf0bZ@google.com>
References: <20220910221947.171557773@linutronix.de>
 <20220910222300.712668210@linutronix.de>
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On (22/09/11 00:27), Thomas Gleixner wrote:
[..]
> + * console_sem protects the console_drivers list, and also provides
> + * serialization for access to the entire console driver system.
> + *
> + * console_mutex serializes register/unregister. console_sem has to be
> + * taken for any list manipulation inside the console_mutex locked
> + * section to keep the console BKL machinery happy.
>   */
> +static DEFINE_MUTEX(console_mutex);
>  static DEFINE_SEMAPHORE(console_sem);
[..]
>  /*
>   * Helper macros to handle lockdep when locking/unlocking console_sem. We use
>   * macros instead of functions so that _RET_IP_ contains useful information.
> @@ -2978,17 +3008,21 @@ struct tty_driver *console_device(int *i
>  void console_stop(struct console *console)
>  {
>  	__pr_flush(console, 1000, true);
> +	console_list_lock();
>  	console_lock();
>  	console->flags &= ~CON_ENABLED;
>  	console_unlock();
> +	console_list_unlock();
>  }
>  EXPORT_SYMBOL(console_stop);
>  
>  void console_start(struct console *console)
>  {
> +	console_list_lock();
>  	console_lock();
>  	console->flags |= CON_ENABLED;
>  	console_unlock();
> +	console_list_unlock();
>  	__pr_flush(console, 1000, true);
>  }
>  EXPORT_SYMBOL(console_start);

So the comment says that list lock (console_mutex) is to serialize
register/unregister, but then we take it in stop/start as well. What
does list lock protect us against in start/stop? console->flags reader
(console_is_usable()) does not take list lock, it's called under console
lock and console->flags writers (console_unregister() and console_stop())
modify console->flags under console_lock.