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    SubjectRe: [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
    > Dipankar Sarma <dipankar@in.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.

    What are you talking about here ? That is the write side. You are
    *not* supposed to do lock_cpu_hotplug() in cpu callbacks paths AFAICT.
    If someone does it (like cpufreq did), it is just *wrong*.

    > > 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 am talking about readsides here - you read cpu_online_map and
    block then reuse the map and make some calls to another subsystem
    that may again do a similar read-side cpu_hotplug lock. I suspect
    that it hard to get rid of all possible dependencies.

    > > > 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.

    Do cpu hotplugs in a one cpu and change frequencies in another -
    I think Gautham has a script to reproduce this. Besides
    lockdep apparently complains about it -

    http://marc.theaimsgroup.com/?l=linux-kernel&m=115359728428432&w=2

    > > 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.

    Really, the hotplug locking rules are fairly simple-

    1. If you are in cpu hotplug callback path, don't take any lock.

    2. If you are in a non-hotplug path reading cpu_online_map and you don't
    block, you just disable preemption and you are safe from hotplug.

    3. If you are in a non-hotplug path and you use cpu_online_map and
    you *really* need to block, you use lock_cpu_hotplug() or
    cpu_hotplug_disable whatever it is called.

    Is this too difficult for people to follow ?

    >
    > > 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.

    No it doesn't. Perhaps we should just document the rules better
    and put some static checks for people to get it right.

    Thanks
    Dipankar
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