On Sun, 27 Aug 2006 11:41:55 +0530
Dipankar Sarma <[email protected]> wrote:
> Now coming to the read-side of lock_cpu_hotplug() - cpu hotplug
> is a very special asynchronous event. You cannot protect against
> it using your own subsystem lock because you don't control
> access to cpu_online_map.
Yes you do. Please, read _cpu_up(), _cpu_down() and the example in
workqueue_cpu_callback(). It's really very simple.
> With multiple low-level subsystems
> needing it, it also becomes difficult to work out the lock
> hierarchies.
That'll matter if we do crappy code. Let's not do that?
> >
> > I rather doubt that anyone will be hitting the races in practice. I'd
> > recommend simply removing all the lock_cpu_hotplug() calls for 2.6.18.
>
> I don't think that is a good idea.
The code's already racy and I don't think anyone has reported a
cpufreq-vs-hotplug race.
> 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).
It needs to be ripped out and some understanding, thought and design should
be applied to the problem.
> People
> seem to have just got lazy with lock_cpu_hotplug().
That's because lock_cpu_hotplug() purports to be some magical thing which
makes all your troubles go away.
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