linux-kernel - Re: linux.git: printk() problem

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Date:   Sun, 23 Oct 2016 13:33:41 -0700
From:   Joe Perches <joe@...ches.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.cz>
Subject: Re: linux.git: printk() problem

On Sun, 2016-10-23 at 12:32 -0700, Linus Torvalds wrote:
> On Sun, Oct 23, 2016 at 12:06 PM, Joe Perches <joe@...ches.com> wrote:
> > On Sun, 2016-10-23 at 11:11 -0700, Linus Torvalds wrote:
> > > 
> > > And those two per se sound fairly easy to handle ("KERN_CONT means
> > > append to the line buffer, otherwise flush the line buffer and move to
> > > the record buffer").
> > > 
> > > But what complicates things more is then the "console output", which
> > > has two issues:
> > > 
> > >  - it is done outside the locking regime for the line buffer and the
> > > record buffer.
> > > 
> > >  - it is done on _partial_ line buffers.
> > 
> > 
> > EOL KERN_<LEVEL> and thread interleaving still exists.
> 
> 
> Note that the thread interleaving is still trivial: it's easily done
> at the point where we decide "can we append to the line buffer or
> not". That's pretty simple. Just flush the record when the thread
> changes.
> 
> So the interleaving will never go away, it's very fundamental - unless
> we make the line buffer just be a per-thread thing. And yes, that
> would be the cleanest solution, but it's also an extra buffer for each
> thread, so realistically it's just not going to happen.

I doubt there are cases where more than a few of
these interleaving threads are simultaneous.

Perhaps it could be a pool of active thread
continuation buffers.  

> End result: I'm not worried about the interleaving. It will cause ugly
> output, but we've always had that, and the solution to it is "if you
> absolutely don't want interleaving, then don't try to print partial
> lines!".
> The classic "don't do that then" response, in other world.

Yup, best solution.