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

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Date:   Mon, 10 Apr 2017 20:48:58 +0200
From:   Pavel Machek <pavel@....cz>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Jan Kara <jack@...e.cz>,
        "Eric W. Biederman" <ebiederm@...ssion.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 Mon 2017-04-10 13:53:39, Sergey Senozhatsky wrote:
> On (04/09/17 12:12), Pavel Machek wrote:
> [..]
> > > a side note,
> > > that's rather unclear to me how would "message delayed" really help.
> > > if your system hard-lockup so badly and there are no printk messages
> > > even from NMI watchdog, then we won't be able to print that message.
> > 
> > We are talking about
> > 
> >    printk("unusual condition");
> >    do_something_clever(); /* Which unfortunately hard-crashes the machine */
> > 
> > that works with my proposal, but not with yours. Seen it happen many
> > times before.
> 
> I see your point, sure.
> I can't completely agree on "that works with my proposal, but not with yours."
> 
> on SMP system this would be true only if no other CPU holds the console_sem
> at the time we call printk(). (skipping irrelevant cases when we have suspended
> console or !online CPU and !CON_ANYTIME console). and there is nothing that
> makes "no other CPU holds the console_sem" always true on SMP system at any
> given point in time. so no, "A always works, B never works" is not
> accurate.

Ok, you are right. OTOH the common case is console_sem is unlocked (at
least on systems I develop on). 

> but, once again, I see your point.

Good. Does that mean that the next version of patches will work ok in
that case?
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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