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

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Message-ID: <20190418000014.GA2059@jagdpanzerIV>
Date:   Thu, 18 Apr 2019 09:00:14 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>, Feng Tang <feng.tang@...el.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.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 (04/17/19 23:18), Feng Tang wrote:
> > > +++ 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;
> 
> This is very similar to my V1 patch :), excepted I used a bool
> as the parameter.

Yes it is :)

I will reply to Petr's and Feng's email.

> > 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.
>
> My thought is, the replay is only a debug option and disabled by default,
> so when user specifically enable the bit of PANIC_PRINT_ALL_PRINTK_MSG,
> the whole replay of printk msg should be expected.

I think that PANIC_PRINT_ALL_PRINTK_MSG is a debugging option; a quite
specific one. So people who ask the kernel to PANIC_PRINT_ALL_PRINTK_MSG
they know what they are doing, and we probably will not cofuse anyone.
After all, we don't print any headers when we ftrace_dump() or imitate
sysrq via sysrq_timer_list_show(), or for any other panic_print_sys_info()
printouts. So it's OK to just do the simple thing for
PANIC_PRINT_ALL_PRINTK_MSG.

	-ss