linux-kernel - Re: [PATCH v2] [watchdog] combine nmi

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Date:	Fri, 16 Apr 2010 16:59:22 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	Don Zickus <dzickus@...hat.com>, mingo@...e.hu,
	aris@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] [watchdog] combine nmi_watchdog and softlockup

On Fri, Apr 16, 2010 at 04:53:11PM +0200, Peter Zijlstra wrote:
> On Fri, 2010-04-16 at 16:46 +0200, Frederic Weisbecker wrote:
> > > > May be have an arch spin lock there to update your cpu mask safely.
> > > > 
> > > 
> > > Hmm, this is NMI handler path so from what we protect this per-cpu data?
> > > Do I miss something? /me confused
> > 
> > 
> > The cpu mask is not per cpu here, this is a shared bitmap, so you
> > can race against other cpus NMIs.
> > 
> > That said, as I suggested, having a per cpu var that we set when we
> > warned would be much better than a spinlock here. 
> 
> Every time you think NMI and spinlock think FAIL.


In fact I was first inspired by the x86 nmi watchdog handler
that does this spinlock to protect cpumask, but realize just
after my FAIL ;-)

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