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Date:   Mon, 26 Sep 2022 10:33:35 +0100
From:   Aaron Tomlin <atomlin@redhat.com>
To:     John Ogness <john.ogness@linutronix.de>
Cc:     Petr Mladek <pmladek@suse.com>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org,
        Jason Wessel <jason.wessel@windriver.com>,
        Daniel Thompson <daniel.thompson@linaro.org>,
        Douglas Anderson <dianders@chromium.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jirislaby@kernel.org>,
        Luis Chamberlain <mcgrof@kernel.org>,
        kgdb-bugreport@lists.sourceforge.net, linux-serial@vger.kernel.org
Subject: Re: [PATCH printk 10/18] kgbd: Pretend that console list walk is safe
Message-ID: <20220926093335.exvrpwp5khlwqxhp@ava.usersys.com>
References: <20220924000454.3319186-1-john.ogness@linutronix.de>
 <20220924000454.3319186-11-john.ogness@linutronix.de>
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In-Reply-To: <20220924000454.3319186-11-john.ogness@linutronix.de>
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On Sat 2022-09-24 02:10 +0206, John Ogness wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
> 
> Provide a special list iterator macro for KGDB to allow unprotected list
> walks and add a few comments to explain the hope based approach.
> 
> Preperatory change for changing the console list to hlist and adding
> lockdep asserts to regular list walks.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: John Ogness <john.ogness@linutronix.de>
> Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
> ---
>  drivers/tty/serial/kgdboc.c |  5 ++++-
>  include/linux/console.h     | 10 ++++++++++
>  kernel/debug/kdb/kdb_io.c   |  7 ++++++-
>  3 files changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c
> index af2aa76bae15..57a5fd27dffe 100644
> --- a/drivers/tty/serial/kgdboc.c
> +++ b/drivers/tty/serial/kgdboc.c
> @@ -462,10 +462,13 @@ static void kgdboc_earlycon_pre_exp_handler(void)
>  	 * we have no other choice so we keep using it.  Since not all
>  	 * serial drivers might be OK with this, print a warning once per
>  	 * boot if we detect this case.
> +	 *
> +	 * Pretend that walking the console list is safe...
>  	 */
> -	for_each_console(con)
> +	for_each_console_kgdb(con) {
>  		if (con == kgdboc_earlycon_io_ops.cons)
>  			return;
> +	}
>  
>  	already_warned = true;
>  	pr_warn("kgdboc_earlycon is still using bootconsole\n");
> diff --git a/include/linux/console.h b/include/linux/console.h
> index 24344f9b0bc1..86a6125512b9 100644
> --- a/include/linux/console.h
> +++ b/include/linux/console.h
> @@ -187,6 +187,16 @@ extern void console_list_unlock(void) __releases(console_mutex);
>  #define for_each_console(con)						\
>  	for (con = console_drivers; con != NULL; con = con->next)
>  
> +/**
> + * for_each_console_kgdb() - Iterator over registered consoles for KGDB
> + * @con:	struct console pointer used as loop cursor
> + *
> + * Has no serialization requirements and KGDB pretends that this is safe.
> + * Don't use outside of the KGDB fairy tale land!
> + */
> +#define for_each_console_kgdb(con)					\
> +	for (con = console_drivers; con != NULL; con = con->next)
> +
>  extern int console_set_on_cmdline;
>  extern struct console *early_console;
>  
> diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
> index 67d3c48a1522..fb3775e61a3b 100644
> --- a/kernel/debug/kdb/kdb_io.c
> +++ b/kernel/debug/kdb/kdb_io.c
> @@ -558,7 +558,12 @@ static void kdb_msg_write(const char *msg, int msg_len)
>  		cp++;
>  	}
>  
> -	for_each_console(c) {
> +	/*
> +	 * This is a completely unprotected list walk designed by the
> +	 * wishful thinking department. See the oops_in_progress comment
> +	 * below - especially the encourage section...
> +	 */
> +	for_each_console_kgdb(c) {
>  		if (!(c->flags & CON_ENABLED))
>  			continue;
>  		if (c == dbg_io_ops->cons)
> -- 
> 2.30.2
> 

Reviewed-by: Aaron Tomlin <atomlin@redhat.com>

-- 
Aaron Tomlin