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

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Message-Id: <20060826234618.b9b2535a.akpm@osdl.org>
Date:	Sat, 26 Aug 2006 23:46:18 -0700
From:	Andrew Morton <akpm@...l.org>
To:	dipankar@...ibm.com
Cc:	Linus Torvalds <torvalds@...l.org>, Dave Jones <davej@...hat.com>,
	ego@...ibm.com, rusty@...tcorp.com.au,
	linux-kernel@...r.kernel.org, arjan@...el.linux.com, mingo@...e.hu,
	vatsa@...ibm.com, ashok.raj@...el.com
Subject: Re: [RFC][PATCH 0/4] Redesign cpu_hotplug locking.

On Sun, 27 Aug 2006 11:41:55 +0530
Dipankar Sarma <dipankar@...ibm.com> 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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