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

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Message-ID: <CAHk-=wgh=iQD2Xb-jRSE45Qm1H=y34OdzG1B-pjMWT1Kr2E=rA@mail.gmail.com>
Date: Tue, 6 Aug 2024 10:29:19 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Petr Mladek <pmladek@...e.com>
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

On Tue, 6 Aug 2024 at 02:59, Petr Mladek <pmladek@...e.com> wrote:
>
> I want to be sure that this is acceptable to you.

Just going by the superficial code, it's perfectly fine to delay other
consoles, as long as *some* consoles don't get delayed.

So yes:

> It is a bit different from the buffering during Oops. In this case,
> the new nbcon consoles will still be flushed immediately.

that sounds fine to me.

In general, I think delaying flushing is fine if

 (a) re-entrancy issues with already active (ie non-buffered) printing
on a console

 (b) there are higher-priority consoles that did flush it so that
something made it out

and obviously the whole "nbcon existing" will match that (b) case.

(In a perfect world the (a) case would be per-cpu, ie a "avoid obvious
deadlock", but honestly, I think any oops while the console is already
busy synchronously printing is understandable - and any concurrent
synchronous printing may be the more immediate cause of the oops -
we've certainly seen lots of cases where a bug on one CPU will then
immediately trigger on another one).

            Linus