Re: printk: what is going on with additional newlines?

[Sergey Senozhatsky]

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