linux-kernel - Re: [PATCH] printk: inject caller information into the body of message

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Date:   Wed, 20 Jun 2018 20:07:59 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        syzkaller <syzkaller@...glegroups.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Fengguang Wu <fengguang.wu@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] printk: inject caller information into the body of
 message

On (06/20/18 11:31), Dmitry Vyukov wrote:
> > BTW, pr_cont() handling is not so simple when we are in printk_safe()
> > context. Unlike vprintk_emit() [normal printk], we don't use any
> > dedicated pr_cont() buffer in printk_safe. So, at a glance, I suspect
> > that injecting context info at every printk_safe_log_store() call for
> > `for (...) pr_cont()' loop is going to produce something like this:
> >         I<10> 23 I<10> 43 I<10> 47 ....
> >
> >         // Hmm, maybe the line will endup having two prefixes. Once
> >         // from printk_safe_log_store, the other from normal printk
> >         // log_store().
> >
> > While the same `for (...) pr_cont()' called from normal printk() context
> > will produce
> >         I<10> 32 43 47 ....
> >
> > It could be that I'm wrong.
> > Tetsuo, have you tested pr_cont() from printk_safe() context?
> 
> 
> So this is another reason to get rid of pr_cont entirely, right?

Getting rid of pr_cont() from important output would be totally cool.
Quoting Linus:

    Only acceptable use of continuations is basically boot-time testing,
    when you do things like

     printk("Testing feature XYZ..");
     this_may_blow_up_because_of_hw_bugs();
     printk(KERN_CONT " ... ok\n");


I can recall at least 4 attempts when people tried to introduce new pr_cont()
or some concept with similar functionality to pr_cont(), but SMP safe. We
brought the first one - per-CPU pr_cont() buffers - to KS several years ago
but Linus didn't like it. Then there was a buffered printk() mode patch from
Tetsuo, then a solution from Steven, then I had my second try with a
soft-of-pr_cont() replacement.

So, if we could get rid of pr_cont() from the most important parts
(instruction dumps, etc) then I would just vote to leave pr_cont()
alone and avoid any handling of it in printk context tracking. Simply
because we wouldn't care about pr_cont(). This also could simplify
Tetsuo's patch significantly.

	-ss