linux-kernel - Re: [PATCH printk v1 16/18] kernel/panic: Add atomic write enforcement to warn/panic

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Date:   Fri, 14 Apr 2023 12:10:02 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        David Gow <davidgow@...gle.com>,
        Tiezhu Yang <yangtiezhu@...ngson.cn>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        tangmeng <tangmeng@...ontech.com>
Subject: Re: [PATCH printk v1 16/18] kernel/panic: Add atomic write
 enforcement to warn/panic

On Thu 2023-04-13 14:19:13, John Ogness wrote:
> On 2023-04-13, Petr Mladek <pmladek@...e.com> wrote:
> >> --- a/kernel/panic.c
> >> +++ b/kernel/panic.c
> >> @@ -329,6 +332,8 @@ void panic(const char *fmt, ...)
> >>  	if (_crash_kexec_post_notifiers)
> >>  		__crash_kexec(NULL);
> >>  
> >> +	cons_atomic_flush(NULL, true);
> >
> > Do we need to explicitly flush the messages here?
> 
> This is where the atomic printing actually starts (after the full dump
> has been inserted into the ringbuffer).
> 
> > cons_atomic_flush() is called also from vprintk_emit(). And there are
> > many messages printed with the PANIC priority above.
> 
> vprintk_emit() does not print in this case. From cons_atomic_flush():
> 
>         /*
>          * When in an elevated priority, the printk() calls are not
>          * individually flushed. This is to allow the full output to
>          * be dumped to the ringbuffer before starting with printing
>          * the backlog.
>          */
>         if (cpu_state->prio > NBCON_PRIO_NORMAL && printk_caller_wctxt)
>                 return;

OK, what is the motivation for this behavior, please?
Does it has any advantages?

> 
> > This makes an assumption that either printk() in PANIC context
> > does not try to show the messages immediately or that this
> > explicit console_atomic_flush() tries harder. I think
> > that both assumptions are wrong.
> 
> Both assumptions are correct, because until this point there has been no
> effort to print.

Honestly, this makes me nervous. It means that panic() messages will
not reach the console unless they are explicitly flushed.

First, it is error-prone because it requires calling
console_atomic_flush() in all relevant code paths on the right
locations.

Second, it expects that panic() code could never fail between
the explicit console_atomic_flush() calls. If it failed, it might
be pretty useful to see the last printed message.

Third, messages might get lost when there are too many. And it is
realistic. For example, see panic_print_sys_info() it might add
quite long reports.

I would really prefer to flush atomic consoles with every printk()
unless there is a really good reason not to do it.

Best Regards,
Petr