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