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

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Date:   Mon, 10 Apr 2017 13:53:39 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Pavel Machek <pavel@....cz>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...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 (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.

but, once again, I see your point.

	-ss