Re: linux.git: printk() problem

From: Linus Torvalds
Date: Wed Oct 12 2016 - 11:47:15 EST

On Wed, Oct 12, 2016 at 6:30 AM, Tetsuo Handa
<penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
> I noticed that current linux.git generates hardly readable console output
> due to KERN_CONT changes. Are you suggesting developers that output like
> this be fixed?

Yes. Needing to add a few KERN_CONT markers was pretty much expected,
since it's about 5 years since we enfroced it and new code won't
necessarily have it (and even back then I don't think we _always_ got
it right).

That said, looking at the printk's in the lowmem code, I think it
could be useful if there was some effort to see if the code could
somehow avoid the multi-printk thing. This is actually one area where

(a) the problem actually happens while the system is running, rather
than booting

(b) I've seen line mixing in the past

but the short term fix is to just add KERN_CONT markers to the lines
that are continuations.

NOTE! The reason I mention that (a) thing that it has traditionally
made it much messier to do logging of continuation lines in the first
place (because more things are going on and often one problem leads to
another and then the mixing is much more likely), but I actually
intentionally made it more likely to trigger the flushing issue in
commit bfd8d3f23b51 ("printk: make reading the kernel log flush
pending lines").

So if there is an active system logger that is reading messages *when*
one of those "one line in multiple printk's" things happen, that log
reader will now potentially cause the logging to be broken up because
the act of reading will flush the pending lines.

Now, honestly, that is something that we may end up reverting, but I'd
_like_ to try not to. Because without that flushing, there might be
one last partial line that the logger never sees. So it was me trying
to be aggressive about those partial lines, and the *hope* is that we
can just keep it, and that we can look at areas that have problems
with it.

We'll see. But the other issues are easily fixed by just adding
KERN_CONT where appropriate. It was actually very much what you were
supposed to do before too, if only as a marker to others that "yes,
I'm actually doing this, and no, I'm not supposed to have a log level"
(but with the new world order you actually *can* combine KERN_CONT
with a loglevel, so that if the beginning od the line got flushed, the
continuation can still be printed with the right log level).