linux-kernel - Re: printk: what is going on with additional newlines?

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Date:   Tue, 29 Aug 2017 19:50:13 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Pavel Machek <pavel@....cz>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>, Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jiri Slaby <jslaby@...e.com>, Andreas Mohr <andi@...as.de>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: printk: what is going on with additional newlines?

On Tue, 29 Aug 2017 10:12:22 -0700
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Tue, Aug 29, 2017 at 10:00 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > I refuse to help those things. We mis-designed things  
> 
> Actually, let me rephrase that:
> 
> It might actually be a good idea to help those things, by making
> helper functions available that do the marshalling.
> 
> So not calling "printk()" directly, but having a set of simple
> "buffer_print()" functions where each user has its own buffer, and
> then the "buffer_print()" functions will help people do nicely output
> data.
> 
> So if the issue is that people want to print (for example) hex dumps
> one character at a time, but don't want to have each character show up
> on a line of their own, I think we might well add a few functions to
> help dop that.
> 
> But they wouldn't be "printk". They would be the buffering functions
> that then call printk when tyhey have buffered a line.
> 
> That avoids the whole nasty issue with printk - printk wants to show
> stuff early (because _maybe_ it's critical) and printk wants to make
> log records with timestamps and loglevels. And printk has serious
> locking issues that are really nasty and fundamental.
> 
> A private buffer has none of those issues.

What about using the seq_buf*() then?

	struct seq_buf s;

	buf = kmalloc(mysize);
	seq_buf_init(&s, buf, mysize);

	seq_printf(&s,"blah blah %d", bah_blah);
	[...]
	seq_printf(&s, "my last print\n");

	printk("%.*s", s.len, s.buffer);

	kfree(buf);

This is what the NMI "safe" printks basically do.

-- Steve