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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Wed, 20 Jun 2018 13:32:07 +0200
Message-ID: <CACT4Y+aER3O2x2ApM=MVFoFXhp_T_rUeyG9CBx031qyH0voSRg@mail.gmail.com>
Subject: Re: [PATCH] printk: inject caller information into the body of message
To:     Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc:     Petr Mladek <pmladek@suse.com>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        syzkaller <syzkaller@googlegroups.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Fengguang Wu <fengguang.wu@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>
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On Wed, Jun 20, 2018 at 1:07 PM, Sergey Senozhatsky
<sergey.senozhatsky.work@gmail.com> wrote:
> 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.

Sounds good to me.