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

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Date:   Thu, 13 Aug 2020 20:54:35 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...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 10:41), Petr Mladek wrote:
> > My concerns about this idea:
> > 
> > - What if the printk user does not correctly terminate the cont message?
> >   There is no mechanism to allow that open record to be force-finalized
> >   so that readers can read newer records.
> 
> This is a real problem. And it is the reason why the cont buffer is
> currently flushed (finalized) by the next message from another context.

I understand that you think that this should be discussed and addressed
later in a separate patch, but, since we are on pr_cont topic right now,
can we slow down and maybe re-think what is actually expected from
pr_cont()? IOW, have the "what is expect from this feature" thread?

For instance, is missing \n the one and only reason why printk-s from
another context flush cont buffer now? Because I can see some more reasons
for current behaviour and I'd like to question those reasons.

I think what Linus said a long time ago was that the initial purpose of
pr_cont was

	pr_info("Initialize feature foo...");
	if (init_feature_foo() == 0)
		pr_cont("ok\n");
	else
		pr_cont("not ok\n");

	And if init_feature_foo() crashes the kernel then the first printk()
	form panic() will flush the cont buffer.

We can handle this by realizing that new printk() message has LOG_NEWLINE
and has different log_level (not pr_cont), maybe.

Let's look at the more general case:

CPU0					.. CPU255
pr_info("text");
					pr_alert("boom\n");
pr_cont("1);
pr_cont("2\n");

Do we really need to preliminary flush CPU0 pr_cont buffer in this
case? There is no connection between messages from CPU0 and CPU255.
Maybe (maybe!) what matters here is keeping the order of messages
per-context rather than globally system-wide?

	-ss