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

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Message-ID: <b87a02b75c715d9a9f381fe3869612bb826a7351.camel@perches.com>
Date:   Fri, 14 Aug 2020 16:52:24 -0700
From:   Joe Perches <joe@...ches.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        John Ogness <john.ogness@...utronix.de>,
        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 Fri, 2020-08-14 at 15:46 -0700, Linus Torvalds wrote:
> On Thu, Aug 13, 2020 at 4:54 AM Sergey Senozhatsky
> <sergey.senozhatsky@...il.com> wrote:
> > 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.
> 
> Right.
> 
> This is why I think any discussion that says "people should buffer
> their lines themselves and we should get rid if pr_cont()" is
> fundamentally broken.
> 
> Don't go down that hole. I won't take it. It's wrong.

I don't think it's wrong per se.

It's reasonable to avoid pr_cont when appropriate.

Trivial buffering, or adding and using YA vsprintf 
extension can avoid unnecessary message interleaving.

For instance, I just sent this patch to allow removal
of print_vma_addr and its use of pr_cont.

https://lore.kernel.org/lkml/09f11651f0e913e159b955ac447cd8cadf36cb0d.camel@perches.com/

This is similar to the dump_flags_names removal back
in commit edf14cdbf9a0 ("mm, printk: introduce new format
string for flags")

> The fact is, pr_cont() goes back to the original kernel. No, it wasn't
> pr_cont() back then, and no, there were no actual explicit markers for
> "this is a continuation" at all, it was all just "the last printk
> didn't have a newline, so we continue where we left off".
> 
> We've added pr_cont (and KERN_CONT) since then, and I realize that a
> lot of people hate the complexity it introduces, but it's a
> fundamental complexity that you have to live with.
> 
> If you can't live with pr_cont(), you shouldn't be working on
> printk(), and find some other area of the kernel that you _can_ live
> with.
> 
> It really is that simple.
>