linux-kernel - Re: [PATCH printk v7 29/35] printk: Coordinate direct printing in panic

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Message-ID: <ZrH0CYDseR1b-QfI@pathway.suse.cz>
Date: Tue, 6 Aug 2024 11:59:37 +0200
From: Petr Mladek <pmladek@...e.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: John Ogness <john.ogness@...utronix.de>,
	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>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jani Nikula <jani.nikula@...el.com>,
	Uros Bizjak <ubizjak@...il.com>,
	Joel Granados <j.granados@...sung.com>
Subject: Re: [PATCH printk v7 29/35] printk: Coordinate direct printing in
 panic

Hi Linus,

On Sun 2024-08-04 02:57:32, John Ogness wrote:
> If legacy and nbcon consoles are registered and the nbcon
> consoles are allowed to flush (i.e. no boot consoles
> registered), the legacy consoles will no longer perform
> direct printing on the panic CPU until after the backtrace
> has been stored. This will give the safe nbcon consoles a
> chance to print the panic messages before allowing the
> unsafe legacy consoles to print.
> 
> If no nbcon consoles are registered or they are not allowed
> to flush, there is no change in behavior (i.e. legacy
> consoles will always attempt to print from the printk()
> caller context).

I want to be sure that this is acceptable to you.

This behavior has already been in the rejected pull request
for 6.11, see https://lore.kernel.org/r/Zp-_7R49fIHgIhaq@pathway.suse.cz

You did not complain about this particular change. But it is
yet another buffering in critical situation which you do not
like in general.

It is a bit different from the buffering during Oops. In this case,
the new nbcon consoles will still be flushed immediately. And
the legacy consoles will be blocked only when there is a nbcon
console.

The buffering here should increase the chance to see the messages
on the more safe (nbcon) consoles. The legacy consoles are less
safe primary because of the bust_spinlocks(1) called earlier.

I personally do not have strong opinion. The change makes sense.
It looks like it should make more good than harm. But it is not
a clear win.

Best Regards,
Petr