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

Pavel Machek
2017-09-08T10:18:04Z

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On Wed 2017-08-30 14:37:34, Sergey Senozhatsky wrote:
> 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.

For oopses... please don't. It is quite important that Oops goes out
"as soon as possible". I have seen oopses cut in half, etc... They are
still quite helpful.
Pavel
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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