linux-kernel - Re: nmi_watchdog suspicious

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Message-ID: <20080618163905.GB7117@cvg>
Date:	Wed, 18 Jun 2008 20:39:05 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	"Maciej W. Rozycki" <macro@...ux-mips.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: nmi_watchdog suspicious

[Maciej W. Rozycki - Wed, Jun 18, 2008 at 05:06:29PM +0100]
| On Tue, 17 Jun 2008, Cyrill Gorcunov wrote:
| 
| > Thanks a lot Maciej for comments! I've marked them. I'm not sure but it seems
| > I wrote a bit unclear /my english bad indeed/ ;) I mean - this say 'slipping'
| > (ie useless code executions) _was_ before the patch applied. Now it doesn't
| > slip on this since we do mention explicitly in which case there should be
| > alert counters reset. Other then that - will try to handle your notes. Thanks!
| 
|  It will happen regardless if touch_nmi_watchdog() is called before the
| NMI watchdog has been set up in setup_nmi() or lapic_watchdog_init().  It
| may also happen during that window if an NMI is signalled without any NMI
| status bits set in the Port B register at 0x61 -- in theory that should
| not happen except for a broken configuration, but reality out there seems
| to be quite creative about breakage.
| 
|   Maciej
| 

Maciej, it seems we are talking about different code snippets ;)
I'm talking only about touch_nmi_watchdog(). By now (in -tip tree
we have)

void touch_nmi_watchdog(void)
{
	if (nmi_watchdog == NMI_LOCAL_APIC ||
		nmi_watchdog == NMI_IO_APIC) {
		unsigned cpu;
		...

so we check explicitly the values (so if touch_nmi_watchdog
was called when nmi_watchdog = 0 or -1U this code will not
be executed anyway). So I think I'm a bit lost, Maciej... I just
can't figure out what is wrong with this code, so please help
me ;). If you're talking about apic code in _general_ design
then...well, I think I need some time to _understand_ the code
say byte-by-byte first.

		- Cyrill -
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