linux-kernel - Re: nmi_watchdog suspicious

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Message-ID: <20080616170000.GC7273@cvg>
Date:	Mon, 16 Jun 2008 21:00:00 +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 - Mon, Jun 16, 2008 at 12:49:00AM +0100]
| On Tue, 10 Jun 2008, Cyrill Gorcunov wrote:
| 
| > On 64bit mode nmi_watchdog=NMI_NONE by default (in case if APIC enabled).
| > On 32bit mode nmi_watchdog=NMI_DEFAULT was by default (in any case,
| > but could be set to NMI_NONE in check_timer(), but we don't take
| > this case now).
| 
|  I haven't tracked the 64-bit port, but for plain i386 the watchdog used
| to be on by default, then proved problematic with too many broken pieces
| of equipment (typically because of bugs in the SMM firmware) and thus set
| to off.
| 
| > So lets take a look on touch_nmi_watchdog().
| > There is the condition
| > 
| > 	if (nmi_watchdog > 0)
| > 		...tell to reset counters in nmi_watchdog_tick()
| > 
| > this condition is not taken on 64bit mode, but *was* taken on
| > 32bit mode by default! So who was right then? 64bit version or 32bit?
| > 
| > Maciej, could you take a look please? Maybe I just missing figure
| > in general - ie how nmi_watchdog _should_ work.
| 
|  Well, values >= NMI_INVALID are never used, so the condition is correct.  
| It is meant to be positive whenever a working watchdog has been selected.  
| Obviously nmi_watchdog should be a signed int though, so there is a bug
| there.  You better audit all the uses of the variable...
| 
|   Maciej
| 

Maciej, I think nmi_watchdog could (and probably should) be defined as
unsigned. Here my points of why (fix me please if I'm wrong):

- if we remain it as unsigned we could simplify setup_nmi_watchdog() to
  just check for 'if (nmi >= NMI_INVALID)'
- current code does check for NMI_NONE _and_ NMI_DISABLED at once in most
  cases (only the case it dont is - proc_nmi_enabled() wich could be simplified too)
- the only affected of such sign/unsign contention I found is
  touch_nmi_watchdog() for which I suggested the patch (already in Ingo's tip tree)
  http://lkml.org/lkml/2008/6/12/200
  So there could be some 'useless counters resetting' but it could happen for
  quite short time while APIC in initialization phase.

So I think the only problem could be is - simplification. Maybe some checks
should be isolated in helper functions. Will take a look (and of course will
keep community in touch ;)

		- Cyrill -
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