linux-kernel - Re: [printk] fbc14616f4: BUG:kernel_reboot-without-warning_in_test

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Date:   Fri, 7 Apr 2017 09:15:58 +0200
From:   Pavel Machek <pavel@....cz>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Jan Kara <jack@...e.cz>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Ye Xiaolong <xiaolong.ye@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Petr Mladek <pmladek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Len Brown <len.brown@...el.com>,
        linux-kernel@...r.kernel.org, lkp@...org
Subject: Re: [printk]  fbc14616f4:
 BUG:kernel_reboot-without-warning_in_test_stage

On Fri 2017-04-07 13:44:40, Sergey Senozhatsky wrote:
> Hello,
> 
> On (04/06/17 19:33), Pavel Machek wrote:
> > > This patch set gives up part of the printk() reliability for bounded
> > > latency (at least unless we detect we are really in trouble) which is IMHO
> > > a good trade-off for lots of users (and others can just turn this feature
> > > off).
> > 
> > If they can ever realize they were bitten by this feature.
> > 
> > Can we go for different tradeoff?
> > 
> > In console_unlock(), if you detect too much work, print "Too many
> > messages to print, %d bytes delayed" and wake up kernel thread.
> 
> "too many messages" is undefined. console_unlock() can be called from
> IRQ handler or with preemtion disabled, or under spin_lock, or under
> RCU read lock, etc. etc. By the time we decide to wake up printk_kthread
> from console_unlock() it may be already too late.

So lets define "too many messages" as 240 characters. We know printk
worked rather well for us for more than 20 years. Kernel code is used
to printk taking few miliseconds. 

> besides, this does not really address any of the concerns you have
> pointed out in other emails. we might be unable to wake_up printk_kthread
> (because there is a misbehaving higher prio process, or because the
> scheduler is misbehaving, etc. etc.) so the "emergency mode" is still
> here and still requires special handling.

Yeah? So you know modified printk() does not work, that's why
"emergency mode" exists. Unfortunately, you can't rely on fact that
you can detect half-crashed machines by printk levels. You usually
can't.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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