linux-kernel - Re: [PATCH v2] tty: serial: kgdboc: fix mutex locking order for configure

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Date:   Fri, 13 Jan 2023 10:13:59 +0100
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,
        Jason Wessel <jason.wessel@...driver.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Douglas Anderson <dianders@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        kgdb-bugreport@...ts.sourceforge.net, linux-serial@...r.kernel.org
Subject: Re: [PATCH v2] tty: serial: kgdboc: fix mutex locking order for
 configure_kgdboc()

On Thu 2023-01-12 17:18:13, John Ogness wrote:
> Several mutexes are taken while setting up console serial ports. In
> particular, the tty_port->mutex and @console_mutex are taken:
> 
>   serial_pnp_probe
>     serial8250_register_8250_port
>       uart_add_one_port (locks tty_port->mutex)
>         uart_configure_port
>           register_console (locks @console_mutex)
> 
> In order to synchronize kgdb's tty_find_polling_driver() with
> register_console(), commit 6193bc90849a ("tty: serial: kgdboc:
> synchronize tty_find_polling_driver() and register_console()") takes
> the @console_mutex. However, this leads to the following call chain
> (with locking):
> 
>   platform_probe
>     kgdboc_probe
>       configure_kgdboc (locks @console_mutex)
>         tty_find_polling_driver
>           uart_poll_init (locks tty_port->mutex)
>             uart_set_options
> 
> This is clearly deadlock potential due to the reverse lock ordering.
> 
> Since uart_set_options() requires holding @console_mutex in order to
> serialize early initialization of the serial-console lock, take the
> @console_mutex in uart_poll_init() instead of configure_kgdboc().
> 
> Since configure_kgdboc() was using @console_mutex for safe traversal
> of the console list, change it to use the SRCU iterator instead.
> 
> Add comments to uart_set_options() kerneldoc mentioning that it
> requires holding @console_mutex (aka the console_list_lock).
> 
> Fixes: 6193bc90849a ("tty: serial: kgdboc: synchronize tty_find_polling_driver() and register_console()")
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> Reviewed-by: Petr Mladek <pmladek@...e.com>

JFYI, the patch has been committed into printk/linux.git,
branch rework/console-list-lock.

I am going to give it few days in linux-next. If there is no problem
I will send a pull request for 6.2-rc5 later the following week.

Please, let me known if you have another preference.

Best Regards,
Petr