linux-kernel - Re: [PATCH printk v4 13/39] kdb: use srcu console list iterator

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Message-ID: <20221114211647.bml74abrsatcnmda@ava.usersys.com>
Date:   Mon, 14 Nov 2022 21:16:47 +0000
From:   Aaron Tomlin <atomlin@...hat.com>
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>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Douglas Anderson <dianders@...omium.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        kgdb-bugreport@...ts.sourceforge.net
Subject: Re: [PATCH printk v4 13/39] kdb: use srcu console list iterator

On Mon 2022-11-14 17:35 +0106, John Ogness wrote:
> Guarantee safe iteration of the console list by using SRCU.
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> Reviewed-by: Petr Mladek <pmladek@...e.com>
> ---
>  kernel/debug/kdb/kdb_io.c | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
> index 67d3c48a1522..5c7e9ba7cd6b 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)
> @@ -558,8 +559,20 @@ static void kdb_msg_write(const char *msg, int msg_len)
>  		cp++;
>  	}
>  
> -	for_each_console(c) {
> -		if (!(c->flags & CON_ENABLED))
> +	/*
> +	 * 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.
> +	 *
> +	 * There is no guarantee that every console driver 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.
> +	 */
> +	cookie = console_srcu_read_lock();
> +	for_each_console_srcu(c) {
> +		if (!(console_srcu_read_flags(c) & CON_ENABLED))
>  			continue;
>  		if (c == dbg_io_ops->cons)
>  			continue;
> @@ -577,6 +590,7 @@ 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
> 

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

-- 
Aaron Tomlin