linux-kernel - Re: POC: Alternative solution: Re: [PATCH 0/4] printk: reimplement LOG

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Date:   Thu, 13 Aug 2020 14:18:53 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        kexec@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: POC: Alternative solution: Re: [PATCH 0/4] printk: reimplement
 LOG_CONT handling

On (20/08/13 02:30), John Ogness wrote:
> 2. I haven't yet figured out how to preserve calling context when a
> newline appears. For example:
> 
> pr_info("text");
> pr_cont(" 1");
> pr_cont(" 2\n");
> pr_cont("3");
> pr_cont(" 4\n");
>
> For "3" the calling context (info, timestamp) is lost because with "2"
> the record is finalized. Perhaps the above is invalid usage of LOG_CONT?

This is not an unseen pattern, I'm afraid. And the problem here can
be more general:

	pr_info("text");
	pr_cont("1");
	exception/IRQ/NMI
		pr_alert("text\n");
	pr_cont("2");
	pr_cont("\n");

I guess the solution would be to store "last log_level" in task_struct
and get current (new) timestamp for broken cont line?

We have this problem now. E.g. early boot messages from one of my boxes:

6,173,41837,-;x86: Booting SMP configuration:
6,174,41838,-;.... node  #0, CPUs:      #1 #2 #3 #4
4,175,44993,-;MDS CPU bug present and SMT on, data leak possible. See https://www.kernel.org/doc/...
4,176,44993,c; #5 #6 #7

"CPUs: #1 #2 #3 #4 #5 #6 #7" is supposed to be one cont line with
loglevel 6. But it gets interrupted and flushed so that "#5 #6 #7"
appears with the different loglevel.

	-ss