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

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Date:   Fri, 13 Jan 2023 11:32:41 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        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] tty: serial: kgdboc: fix mutex locking order for
 configure_kgdboc()

On (23/01/12 16:40), Petr Mladek wrote:
> On Wed 2023-01-11 17:23:55, Petr Mladek wrote:
> > On Wed 2023-01-11 15:57:10, 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.
[..]
> > > -	console_list_lock();
> > > -
> > >  	p = tty_find_polling_driver(cptr, &tty_line);
> > >  	if (!p) {
> > >  		console_list_unlock();
> > 
> > This console_list_unlock() should be removed here as well.
> 
> This seems to be the only problem. Otherwise, the patch looks good to
> me.

Good catch.

Reviewed-by: Sergey Senozhatsky <senozhatsky@...omium.org>