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

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Message-ID: <20120626002307.GA4389@kroah.com>
Date:	Mon, 25 Jun 2012 17:23:07 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Linus Torvalds <torvalds@...ux-foundation.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 05:01:11PM -0700, Linus Torvalds wrote:
> On Mon, Jun 25, 2012 at 4:55 PM, Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> wrote:
> >
> > Stephen and Ingo, I understand that your tests now would require
> > multiple printk() lines, but this affects what, 10 boxes in the world
> > that run these tests (I'm not trying to be mean, just understand the
> > issues).  The fixes that now are in place fix problems for many more
> > systems, and provide the infrastructure for proper logging that people
> > have been screaming at us for over 10 years to accomplish.
> 
> I disagree violently.
> 
> I think we absolutely should apply Steven's patch.
> 
> Why? Because the buffering does not help *anything*, and it's
> surprising, and it breaks one of our main debugging tools. There's no
> upside to it.

Ok, but I thought you wanted the "properly handle continuations" that we
now have in the kernel.  I must be mistaken.

> The fact that we found *one* case where it broke within days of it
> being introduced is not the issue. Fixing that one case is irrelevant.
> It's the unknown number of other cases that did similar thngs that
> matter.
> 
> If there are other places that print out partial lines, they may have
> this problem too. Don't buffer.
> 
> And if there are *not* other places that print out partial lines, then
> buffering doesn't help. Don't buffer.
> 
> Notice? Buffering partial lines is never *ever* the right thing to do
> for something like printk.
> 
> If you want to merge the partial lines, do it at the *logging* stage,
> not at the printout stage. Nobody cares if you buffer the stuff that
> actually makes it to "dmesg". But buffering the stuff before it makes
> it to the screen is just wrong.

Ok, Kay, does Stephen's patch work for you as well?  I'll go boot a box
with it and look at the output, but that will take 30 minutes or so...

greg k-h
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