linux-kernel - Re: [PATCH 3/3] lockdep: Use line-buffered printk() for lockdep messages.

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Message-ID: <20181130160147.r6idgsr2biy5rwap@pathway.suse.cz>
Date:   Fri, 30 Nov 2018 17:01:47 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     David Laight <David.Laight@...LAB.COM>,
        'Steven Rostedt' <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH 3/3] lockdep: Use line-buffered printk() for lockdep
 messages.

On Thu 2018-11-29 19:09:26, Tetsuo Handa wrote:
> On 2018/11/28 22:29, David Laight wrote:
> > I also spent a week trying to work out why a customer kernel was
> > locking up - only to finally find out that the distro they were
> > using set 'panic on opps' - making it almost impossible to find
> > out what was happening.

Did the machine rebooted before the messages reached console or
did it produced crash-dump or frozen?

panic() tries relatively hard to flush the messages to the console,
see printk_safe_flush_on_panic() and console_flush_on_panic().
It is less aggressive when crashdump is called. It might deadlock
in console drivers.

Hmm, it might also fail when another CPU is still running and
steals console_lock. We might want to disable
console_trylock_spinning() if the caller is not
panic_cpu.


> On 2018/11/26 13:34, Sergey Senozhatsky wrote:
> > Or... Instead.
> > We can just leave pr_cont() alone for now. And make it possible to
> > reconstruct messages - IOW, inject some info to printk messages. We
> > do this at Samsung (inject CPU number at the beginning of every
> > message. `cat serial.0 | grep "\[1\]"` to grep for all messages from
> > CPU1). Probably this would be the simplest thing.
> 
> Yes, I sent a patch which helps reconstructing messages at
> http://lkml.kernel.org/r/1543045075-3008-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp .

All the buffering approaches have problems that cannot be solved
easily. The prefix-based approach looks like the best alternative
at the moment.

Best Regards,
Petr