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

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Date:   Fri, 7 Apr 2017 13:44:40 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Pavel Machek <pavel@....cz>
Cc:     Jan Kara <jack@...e.cz>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Ye Xiaolong <xiaolong.ye@...el.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.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

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.

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.

	-ss