linux-kernel - Re: Problem with percpu values when bringing up second CPU?

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Message-Id: <200908051101.20471.rusty@rustcorp.com.au>
Date:	Wed, 5 Aug 2009 11:01:19 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Problem with percpu values when bringing up second CPU?

On Wed, 5 Aug 2009 08:28:34 am Jeremy Fitzhardinge wrote:
> Hi,
> 
> I just tracked down a bug I was having to a change where I changed one
> of my Xen event channel variables to a percpu variable, relating to
> masking an event channel.
> 
> The symptom was that shortly after bringing up the second CPU, the first
> CPU's timer events stopped arriving, apparently because they had become
> masked.
> 
> The event channels masks are declared as:
> 
> #define NR_EVENT_CHANNEL_LONGS (NR_EVENT_CHANNELS/BITS_PER_LONG)
> static DEFINE_PER_CPU(unsigned long,
>                      cpu_evtchn_mask[NR_EVENT_CHANNEL_LONGS]) =
>        {[0 ... NR_EVENT_CHANNEL_LONGS-1] = ~0ul };	/* everything masked by default */
> 
> 
> My theory about what's happening is that when the second CPU comes up,
> it allocates separate percpu areas for each CPU, but it is somehow
> failing to accurately copy CPU 0's percpu data over

If you touch the per-cpu vars before setup_per_cpu_areas(), you will hit the
master copy.

Is that possible?
Rusty.
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