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

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Date:   Fri, 23 Nov 2018 13:46:47 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        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,
        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 Sat 2018-11-10 11:42:03, Tetsuo Handa wrote:
> On 2018/11/10 0:43, Petr Mladek wrote:
> >> + * Line buffered printk() tries to assign a buffer when printk() from a new
> >> + * context identifier comes in. And it automatically releases that buffer when
> >> + * one of three conditions listed below became true.
> >> + *
> >> + *   (1) printk() from that context identifier emitted '\n' as the last
> >> + *       character of output.
> >> + *   (2) printk() from that context identifier tried to print a too long line
> >> + *       which cannot be stored into a buffer.
> >> + *   (3) printk() from a new context identifier noticed that some context
> >> + *       identifier is reserving a buffer for more than 10 seconds without
> >> + *       emitting '\n'.
> >> + *
> >> + * Since (3) is based on a heuristic that somebody forgot to emit '\n' as the
> >> + * last character of output(), pr_cont()/KERN_CONT users are expected to emit
> >> + * '\n' within 10 seconds even if they reserved a buffer.
> > 
> > This is my main concern about this approach. It is so easy to omit
> > the final '\n'.
> 
> If it is so easy to forget the final '\n', there will be a lot of implicit
> pr_cont() users (because pr_cont() assumes that previous printk() omitted the
> final '\n'), and "I am going to investigate much more pr_cont() users." will
> be insufficient for getting meaningful conclusion.
> 
> Checking "lack of the the final '\n'" means that we need to check
> "all printk() users who are not emitting the final '\n'" and evaluate
> "whether there is a possibility that subsequent printk() will not be
>  called from that context due to e.g. conditional branches". That is an
> impossible task for anybody, for there might be out-of-tree code doing it.
> > 
> > They are currently delayed until another printk(). Even this is bad.
> > Unfortunately we could not setup timer from printk() because it
> > would add more locks into the game.
> 
> We could use interval timer for flushing incomplete line.

I am more and more wondering if the buffered printk is worth
the effort. The more buffers we use the more we risk that nobody
would see some important message. Even a part of the line
might be crucial in some situations.

Steven told me on Plumbers conference that even few initial
characters saved him a day few times.


> But updating printk() users to always end with '\n' will be preferable.

This sounds like a whack a mole game. If I get it correctly, you write
that it is "an impossible task for anybody" just few lines above.

Best Regards,
Petr