linux-kernel - Re: [PATCH printk v3 15/40] kdb: use srcu console list iterator

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Message-ID: <20221109085325.wiub564iqnewvczb@ash.lan>
Date:   Wed, 9 Nov 2022 08:53:25 +0000
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        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>,
        Douglas Anderson <dianders@...omium.org>,
        Aaron Tomlin <atomlin@...hat.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        kgdb-bugreport@...ts.sourceforge.net
Subject: Re: [PATCH printk v3 15/40] kdb: use srcu console list iterator

On Mon, Nov 07, 2022 at 03:22:13PM +0106, John Ogness wrote:
> Guarantee safe iteration of the console list by using SRCU.
>
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> ---
>  kernel/debug/kdb/kdb_io.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
> index 550fe8b456ec..ed8289ce4fcb 100644
> --- a/kernel/debug/kdb/kdb_io.c
> +++ b/kernel/debug/kdb/kdb_io.c
> @@ -545,6 +545,7 @@ static void kdb_msg_write(const char *msg, int msg_len)
>  {
>  	struct console *c;
>  	const char *cp;
> +	int cookie;
>  	int len;
>
>  	if (msg_len == 0)
> @@ -558,7 +559,15 @@ static void kdb_msg_write(const char *msg, int msg_len)
>  		cp++;
>  	}
>
> -	for_each_console(c) {
> +	/*
> +	 * The console_srcu_read_lock() only provides safe console list
> +	 * traversal. The use of the ->write() callback relies on all other
> +	 * CPUs being stopped at the moment and console drivers being able to
> +	 * handle reentrance when @oops_in_progress is set. (Note that there
> +	 * is no guarantee for either criteria.)
> +	 */

The debugger entry protocol does ensure that other CPUs are either
stopped or unresponsive. In the case where the other CPU is unresponsive
(e.g. timed out after being asked to stop) then there is a "real" printk()
issued prior to any of the above interference with the console system to
the developer driving the debugger gets as much clue as we can offer them
about what is going on (typically this is emitted from regular interrupt
context).

Given this comment is part of the debugger code then for the
oops_in_progress hack it might be more helpful to describe what
the developer in front the debugger needs to do to have the most
reliable debug session possible.

  There is no guarantee that every console drivers can handle reentrance
  in this way; the developer deploying the debugger is responsible for
  ensuring that the console drivers they have selected handle reentrance
  appropriately.


Daniel.


> +	cookie = console_srcu_read_lock();
> +	for_each_console_srcu(c) {
>  		if (!console_is_enabled(c))
>  			continue;
>  		if (c == dbg_io_ops->cons)
> @@ -577,6 +586,7 @@ static void kdb_msg_write(const char *msg, int msg_len)
>  		--oops_in_progress;
>  		touch_nmi_watchdog();
>  	}
> +	console_srcu_read_unlock(cookie);
>  }
>
>  int vkdb_printf(enum kdb_msgsrc src, const char *fmt, va_list ap)
> --
> 2.30.2
>