linux-kernel - Re: [PATCH 4.19 regression fix] printk: For early boot messages check loglevel when flushing the buffer

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Date:   Tue, 11 Sep 2018 11:30:39 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Maninder Singh <maninder1.s@...sung.com>
Subject: Re: [PATCH 4.19 regression fix] printk: For early boot messages
 check loglevel when flushing the buffer

On (09/10/18 16:57), Petr Mladek wrote:
> 
> Good catch.
> 
> > ---
> > 
> > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > index c036f128cdc3..ede29a7ba6db 100644
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -2545,6 +2545,7 @@ void console_flush_on_panic(void)
> >  	 * ensure may_schedule is cleared.
> >  	 */
> >  	console_trylock();
> > +	exclusive_console = NULL;
> 
> This is not be enough. It would cause replying old messages
> on all consoles.

Oh, that was intentional. I consider repeated messages to be less
problematic than the missing ones.

> Most problems should probably be solved when we store console_seq
> before setting exclusive_console. Then we could clear
> exclusive_console when reaching the stored sequence number.
> 
> Can this be that simple? ;-)

This can work, yes.

I also thought about doing it the way Linus, Jan Kara and Hannes
Reinecke proposed:

- store the console_seq nr of the first oops_in_progress message
  (oops_console_seq) and flush only messages that are in
  [oops_console_seq - 200, log_next_seq] range, as opposed to complete
  logbuf flush.

Hannes asked for this several times. And it was in Jan's printk patches
long time ago (if I'm not mistaken - sorry if I am -- Jan said that Linus
wanted that "just N messages prior to oops" thing).

Jan's patch:
 https://lore.kernel.org/lkml/1457964820-4642-3-git-send-email-sergey.senozhatsky@gmail.com/T/#u


> This reverts commit 375899cddcbb26881b03cb3fbdcfd600e4e67f4a.
> 
> Reported-by: Hans de Goede <hdegoede@...hat.com>
> Signed-off-by: Petr Mladek <pmladek@...e.com>

Acked-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>

	-ss