linux-kernel - Re: [PATCH v4] panic: add an option to replay all the printk message in buffer

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Message-ID: <20190426074934.seje2tn5p6fsuwaq@pathway.suse.cz>
Date:   Fri, 26 Apr 2019 09:49:34 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org,
        Aaro Koskinen <aaro.koskinen@...ia.com>,
        Kees Cook <keescook@...omium.org>, Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH v4] panic: add an option to replay all the printk message
 in buffer

On Thu 2019-04-25 21:32:17, Feng Tang wrote:
> Currently on panic, kernel will lower the loglevel and print out
> pending printk msg only with console_flush_on_panic().
> 
> Add an option for users to configure the "panic_print" to replay
> all dmesg in buffer, some of which they may have never seen due
> to the loglevel setting, which will help panic debugging .
> 
> @@ -2539,6 +2540,11 @@ void console_flush_on_panic(void)
>  	 */
>  	console_trylock();
>  	console_may_schedule = 0;
> +
> +	if (mode == CONSOLE_REPLAY_ALL) {
> +		console_seq = log_first_seq;
> +		console_idx = log_first_idx;

Ah, log_first_seq and log_first_idx are synchronized by
logbuf_log.

console_flush_on_panic(CONSOLE_REPLAY_ALL) is called
when only one CPU is running but it is not guaranteed.

Therefore we should use:

	if (mode == CONSOLE_REPLAY_ALL) {
		unsigned long flags;

		logbuf_lock_irqsave(flags);
		console_seq = log_first_seq;
		console_idx = log_first_idx;
		logbuf_unlock_irqrestore(flags);
	}

I am really sorry that I have missed it yesterday.

Anyway, with the above locking, feel free to add:

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr