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

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Message-ID: <CAPXgP13u769nSvUdS1iqiVCsxG5nsQd7bNT1=-eNw156U-kPkQ@mail.gmail.com>
Date:	Thu, 5 Jul 2012 10:53:56 +0200
From:	Kay Sievers <kay@...y.org>
To:	Michael Neuling <mikey@...ling.org>
Cc:	linuxppc-dev@...abs.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	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 Thu, Jul 5, 2012 at 10:39 AM, Kay Sievers <kay@...y.org> wrote:
> On Thu, Jul 5, 2012 at 9:03 AM, Michael Neuling <mikey@...ling.org> wrote:
>>> On Mon, 2012-06-25 at 18:40 -0700, Linus Torvalds wrote:
>
>>> > I think it might be a great idea to buffer for logging in order to
>>> > generate one individual buffer record there.
>>> >
>>> > But it needs to be printed as it is generated.
>>>
>>> That's a good idea.
>>>
>>> Something like this could work - only minimally tested at this moment.
>>
>> This breaks some powerpc configs and is in Linus' tree now as
>> 084681d14e.
>>
>> When we have printks without a newline (like show_regs()), it
>> sometimes:
>
> x86 has that a lot too.
>
>> 1) drops the console output for that line (dmesg is fine).  Patch to fix
>>    this below.
>
> That doesn't look right. We should already have put that out to the
> console, and we only want to store it away. Your patch, as expected,
> duplicates all the continuation lines on the console here:
> [    0.674957] hpet0: at MMIO 0xfed00000, IRQs 2, 8, 0
> [    0.674957] hpet0: at MMIO 0xfed00000, IRQs 2, 8, 0
>
>> 2) adds a newline unnecessary to both console and dmesg.  I have no fix
>>    for this currently.
>> Reverting this patch fixes both problems.
>
> Not here. I can't reproduce any of this here, it all looks fine.
>
> Is that possibly some early printk() or other console trickery on ppc
> that produces the issue?

Or, are you really sure this isn't just a race with another printk and
the content got merged with some other line of the output? The added
newline you mention could suggest that.

With the buffering for the console output removed, I see garbled
console output here too (like we always had before the new kmsg) when
continuation printks race against printks from any other process. That
was all gone with the buffering (only two continuation prints would
race, not any other), but we needed to skip buffering the console
output for other, more important reasons.

Kay
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