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

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Date:   Wed, 30 Aug 2017 14:37:34 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Joe Perches <joe@...ches.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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 (08/29/17 19:58), Joe Perches wrote:
> > > 
> > > Why?
> > > 
> > > What's wrong with a simple printk?
> > > It'd still do a log_store.
> > 
> > sure, it will. but in separate logbuf entries, and between two
> > consequent printk calls on the same CPU a lot of stuff can happen:
> 
> I think you don't quite understand how this would work.
> The idea is that the entire concatenated bit would be emitted
> in one go.

may be :)

I was thinking about the way to make it work in similar way with
printk-safe/printk-nmi. basically seq buffer should hold both
continuation and "normal" lines, IMHO. when we emit the buffer
we do something like this

	/* Print line by line. */
	while (c < end) {
		if (*c == '\n') {
			printk_safe_flush_line(start, c - start + 1);
			start = ++c;
			header = true;
			continue;
		}

		/* Handle continuous lines or missing new line. */
		if ((c + 1 < end) && printk_get_level(c)) {
			if (header) {
				c = printk_skip_level(c);
				continue;
			}

			printk_safe_flush_line(start, c - start);
			start = c++;
			header = true;
			continue;
		}

		header = false;
		c++;
	}

except that instead of printk_safe_flush_line() we will call log_store()
and the whole loop will be under logbuf_lock.

for that to work, we need API to require header/loglevel etc for every
message. so the use case can look like this:

	init_printk_buffer(&buf);
	print_line(&buf, KERN_ERR "Oops....\n");

	print_line(&buf, KERN_ERR "continuation line: foo");
	print_line(&buf, KERN_CONT "bar");
	print_line(&buf, KERN_CONT "baz\n");
	...

	print_line(&buf, KERN_ERR "....\n");
	...
	print_line(&buf, KERN_ERR "--- end of oops ---\n");
	emit_printk_buffer(&buf);

so that not only concatenated continuation lines will be handled,
but also more complex things. like backtraces or whatever someone
might want to handle.

	-ss