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

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Date:   Wed, 30 Aug 2017 09:58:59 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Pavel Machek <pavel@....cz>, Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>, 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?

Hello,

On (08/29/17 10:52), Linus Torvalds wrote:
> On Tue, Aug 29, 2017 at 10:33 AM, Sergey Senozhatsky
> <sergey.senozhatsky@...il.com> wrote:
> >
> > ok. that's something several people asked for -- some sort of buffered
> > printk mode; but people don't want to use a buffer allocated on the stack
> > (or kmalloc-ed, etc.) to do sprintf() on it and then feed it to printk("%s"),
> > because this adds some extra cost:
> 
[..]
> Introduce a few helper functions for it:
> 
>  init_line_buffer(&buf);
>  print_line(&buf, fmt, args);
>  vprint_line(&buf, fmt, vararg);
>  finish_line(&buf);
> 
> or whatever, and it sounds like it should be pretty easy to use.

ok, I was short on details (sorry, it was almost 3am).

what I was talking/thinking about is not just a single complete continuation
line, but a whole bunch of printk calls (including continuation lines). like
OOM report with backtraces, and so on. the problem people are having (well,
according to emails I have got in my inbox) is the fact that
	printk("a"); printk("b");
	
can appear in the logbuf (and serial console) pretty far; no one knows what
can happen between those calls. so the buffered-printk buffer is supposed to
be big enough for N lines and, more importantly, it stores those lines in
logbuf in consequent entries.

so the difference here is

	while (buffer->whatever)
		printk("%s\n", buffer->msg[i]);

vs

	spin_lock(&logbuf_lock);
	while (buffer->whatever)
		log_store(buffer->msg[i]);
	spin_unlock(&logbuf_lock);


a dynamic buffer with resizing probably may not work good enough in some
OOM cases.

	-ss