linux-kernel - Re: [RFC][PATCH 0/4] Redesign cpu

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Message-ID: <20060827073730.GE22565@in.ibm.com>
Date:	Sun, 27 Aug 2006 13:07:30 +0530
From:	Dipankar Sarma <dipankar@...ibm.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	Linus Torvalds <torvalds@...l.org>, Dave Jones <davej@...hat.com>,
	ego@...ibm.com, rusty@...tcorp.com.au,
	linux-kernel@...r.kernel.org, arjan@...ux.intel.com, mingo@...e.hu,
	vatsa@...ibm.com, ashok.raj@...el.com
Subject: Re: [RFC][PATCH 0/4] Redesign cpu_hotplug locking.

On Sat, Aug 26, 2006 at 11:46:18PM -0700, Andrew Morton wrote:
> On Sun, 27 Aug 2006 11:41:55 +0530
> 
> > The right thing to do would be to
> > do an audit and clean up the bad lock_cpu_hotplug() calls.
> 
> No, that won't fix it.  For example, take a look at all the *callers* of
> cpufreq_update_policy().  AFAICT they're all buggy.  Fiddling with the
> existing lock_cpu_hotplug() sites won't fix that.  (Possibly this
> particular problem can be fixed by checking that the relevant CPU is still
> online after the appropriate locking has been taken - dunno).
> 

This is a different issue from the ones that relates to lock_cpu_hotplug().
This one seems like a cpufreq internal locking problem.

On a quick look at this, it seems to me that cpufreq_cpu_get() should
do exactly what you said - use a spinlock in each cpufreq_cpu_data[] to
protect the per-cpu flag and in cpufreq_cpu_get() check if
!data and data->online == 0. They may have to do - 

static struct cpufreq_data {
	spinlock_t lock;
	int flag;
	struct cpufreq_policy *policy;
} cpufreq_cpu_data[NR_CPUS];


Thanks
Dipankar
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