linux-kernel - Re: [PATCH v2] panic: add an option to replay all the printk message in buffer

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Date:   Wed, 17 Apr 2019 14:24:58 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Feng Tang <feng.tang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Borislav Petkov <bp@...e.de>, ying.huang@...el.com
Subject: Re: [PATCH v2] panic: add an option to replay all the printk message
 in buffer

On Wed 2019-04-17 19:50:10, Sergey Senozhatsky wrote:
> On (04/17/19 18:46), Sergey Senozhatsky wrote:
> > 
> > Does not look too complex/ugly.
> 
> Looks simpler than adding one more global state to the
> console_unlock() printing loop.
> 
> // Not tested at all //
> 
> diff --git a/kernel/panic.c b/kernel/panic.c
> index cd73af35ec66..50eacfc9bc7e 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -51,6 +51,7 @@ EXPORT_SYMBOL_GPL(panic_timeout);
>  #define PANIC_PRINT_TIMER_INFO		0x00000004
>  #define PANIC_PRINT_LOCK_INFO		0x00000008
>  #define PANIC_PRINT_FTRACE_INFO		0x00000010
> +#define PANIC_PRINT_ALL_PRINTK_MSG	0x00000020
>  unsigned long panic_print;
>  
>  ATOMIC_NOTIFIER_HEAD(panic_notifier_list);
> @@ -148,6 +149,9 @@ static void panic_print_sys_info(void)
>  
>  	if (panic_print & PANIC_PRINT_FTRACE_INFO)
>  		ftrace_dump(DUMP_ALL);
> +
> +	if (panic_print & PANIC_PRINT_ALL_PRINTK_MSG)
> +		console_flush_on_panic(CONSOLE_FLUSH_ALL);

The console must be replayed as the first thing in
panic_print_sys_info(). Otherwise, the original messages
are replaced by the other dumps. Especially ftrace_dump()
might be pretty long.

Also the names of the function and the parameter are misleading.
All messages are already flushed when this is called.

>  }
>  
>  /**
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 17102fd4c136..da60a185dbbb 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2549,6 +2549,14 @@ void console_flush_on_panic(void)
>  	 */
>  	console_trylock();
>  	console_may_schedule = 0;
> +	if (flush_mode == CONSOLE_FLUSH_ALL) {
> +		/*
> +		 * Can be done under logbuf lock, but it's unlikely that
> +		 * we will have any race conditions here.
> +		 */
> +		console_seq = log_first_seq;
> +		console_idx = log_first_idx;

I agree that it is easier. The cost is that the same messages are
printed again without any explanation.

I still think that it would be convenient to write a header line.
It would help to understand the log for any, even 3rd-party, reader.
Also it would help to find the beginning in a very long log.

If the complexity of console_unlock() is the concern, we could
refactor the code, e.g. put this "reset log" code into a separate
function.

Best Regards,
Petr