linux-kernel - Re: [PATCH v5 4/4] kdb: Switch to use safer dbg_io

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Date:   Wed, 3 Jun 2020 10:25:04 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     daniel.thompson@...aro.org, kgdb-bugreport@...ts.sourceforge.net,
        jason.wessel@...driver.com, dianders@...omium.org,
        sergey.senozhatsky@...il.com, gregkh@...uxfoundation.org,
        jslaby@...e.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 4/4] kdb: Switch to use safer dbg_io_ops over console
 APIs

On Wed 2020-06-03 12:52:15, Sumit Garg wrote:
> In kgdb context, calling console handlers aren't safe due to locks used
> in those handlers which could in turn lead to a deadlock. Although, using
> oops_in_progress increases the chance to bypass locks in most console
> handlers but it might not be sufficient enough in case a console uses
> more locks (VT/TTY is good example).
> 
> Currently when a driver provides both polling I/O and a console then kdb
> will output using the console. We can increase robustness by using the
> currently active polling I/O driver (which should be lockless) instead
> of the corresponding console. For several common cases (e.g. an
> embedded system with a single serial port that is used both for console
> output and debugger I/O) this will result in no console handler being
> used.
> 
> In order to achieve this we need to reverse the order of preference to
> use dbg_io_ops (uses polling I/O mode) over console APIs. So we just
> store "struct console" that represents debugger I/O in dbg_io_ops and
> while emitting kdb messages, skip console that matches dbg_io_ops
> console in order to avoid duplicate messages. After this change,
> "is_console" param becomes redundant and hence removed.
> 
> diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c
> index 4139698..6e182aa 100644
> --- a/drivers/tty/serial/kgdboc.c
> +++ b/drivers/tty/serial/kgdboc.c
> @@ -558,6 +557,7 @@ static int __init kgdboc_earlycon_init(char *opt)
>  	}
>  
>  	earlycon = con;
> +	kgdboc_earlycon_io_ops.cons = con;
>  	pr_info("Going to register kgdb with earlycon '%s'\n", con->name);
>  	if (kgdb_register_io_module(&kgdboc_earlycon_io_ops) != 0) {
>  		earlycon = NULL;

Should we clear kgdboc_earlycon_io_ops.cons here when
kgdb_register_io_module() failed?

> diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
> index c62d764..529116b 100644
> --- a/include/linux/kgdb.h
> +++ b/include/linux/kgdb.h
> @@ -276,8 +276,7 @@ struct kgdb_arch {
>   * the I/O driver.
>   * @post_exception: Pointer to a function that will do any cleanup work
>   * for the I/O driver.
> - * @is_console: 1 if the end device is a console 0 if the I/O device is
> - * not a console
> + * @cons: valid if the I/O device is a console; else NULL.
>   */
>  struct kgdb_io {
>  	const char		*name;
> @@ -288,7 +287,7 @@ struct kgdb_io {
>  	void			(*deinit) (void);
>  	void			(*pre_exception) (void);
>  	void			(*post_exception) (void);
> -	int			is_console;
> +	struct console		*cons;

Nit: I would call it "con". The trailing 's' makes me feel that that the
     variable points to an array or list of consoles.

>  };
>  

Otherwise, it looks pretty straightforward.

Best Regards,
Petr