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

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Message-ID: <20100416144624.GG15159@redhat.com>
Date:	Fri, 16 Apr 2010 10:46:24 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>, mingo@...e.hu,
	peterz@...radead.org, aris@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] [watchdog] combine nmi_watchdog and softlockup

On Fri, Apr 16, 2010 at 06:32:32PM +0400, Cyrill Gorcunov wrote:
> On Fri, Apr 16, 2010 at 03:47:14AM +0200, Frederic Weisbecker wrote:
> [...]
> > > +
> > > +/* Callback function for perf event subsystem */
> > > +void watchdog_overflow_callback(struct perf_event *event, int nmi,
> > > +		 struct perf_sample_data *data,
> > > +		 struct pt_regs *regs)
> > > +{
> > > +	int this_cpu = smp_processor_id();
> > > +	unsigned long touch_ts = per_cpu(watchdog_touch_ts, this_cpu);
> > > +
> > > +	if (touch_ts == 0) {
> > > +		__touch_watchdog();
> > > +		return;
> > > +	}
> > > +
> > > +	/* check for a hardlockup
> > > +	 * This is done by making sure our timer interrupt
> > > +	 * is incrementing.  The timer interrupt should have
> > > +	 * fired multiple times before we overflow'd.  If it hasn't
> > > +	 * then this is a good indication the cpu is stuck
> > > +	 */
> > > +	if (is_hardlockup(this_cpu)) {
> > > +		/* only print hardlockups once */
> > > +		if (cpumask_test_cpu(this_cpu, to_cpumask(hardlockup_mask)))
> > > +			return;
> > > +
> > > +		if (hardlockup_panic)
> > > +			panic("Watchdog detected hard LOCKUP on cpu %d", this_cpu);
> > > +		else
> > > +			WARN(1, "Watchdog detected hard LOCKUP on cpu %d", this_cpu);
> > > +
> > > +		cpumask_set_cpu(this_cpu, to_cpumask(hardlockup_mask));
> > 
> > 
> > 
> > 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

It's not per-cpu which is the problem I believe.  If you have multiple
cpus dealing with hardlockups than they can overwrite each other's data.

Cheers,
Don
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