Re: [RFC][PATCH 0/4] Redesign cpu_hotplug locking.

[Dipankar Sarma <dipankar@in.ibm.com>]

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