linux-kernel - Re: [PATCH v3] printk: Have printk() never buffer its data

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Message-ID: <CA+55aFxhYRqcU2EUmcsuLrhgo4FEO6-b+mB57wB3JRiXEp3ZrA@mail.gmail.com>
Date:	Mon, 25 Jun 2012 17:40:51 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	"kay.sievers" <kay.sievers@...y.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Joe Perches <joe@...ches.com>,
	"Paul E. McKenney" <paulmck@...ibm.com>
Subject: Re: [PATCH v3] printk: Have printk() never buffer its data

On Mon, Jun 25, 2012 at 5:23 PM, Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> Ok, but I thought you wanted the "properly handle continuations" that we
> now have in the kernel.  I must be mistaken.

We had that before too. It has nothing to do with merging the data,
and the problem was just an outright *bug* in the initial record-based
implementation. Nothing to do with buffering.

Line continuations should happen if the previous printk didn't end
with a '\n', and if the next one doesn't have a level. That was always
the rule. It's a simple rule, and it works.

Now, we can (and Key did) tweak the rule a little bit for cases that
never actually happen in practice.

So sure, add the "we only continue on the same line if we're the same
process as the last process that did the printk" rule to the above
rule. In practice it really doesn't matter, since partial lines are
rare to begin with. And taking interrupt depth into account is another
nice tweak. But it's a tweak, it's not important. Nobody really cares,
but you can try to do a bit better.

And none of the above has to do with *buffering*. The above rules are
100% the same whether you buffer or not.

Sure, with buffering, you can handle the insane cases and try to get
interspersed partial lines to do the right thing. And Kay did that.
And it turns out to (a) never matter and (b) cause problems for
debugging. So just saying "stop doing it" is the no-brainer solution.
Don't buffer printouts to the display, because bad things may happen
before the buffer is flushed.

                             Linus
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