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

[Petr Mladek]

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 <pmla...@suse.com>

Best Regards,
Petr