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    SubjectRe: [RFC][PATCH 0/4] Redesign cpu_hotplug locking.
    On Sat, Aug 26, 2006 at 03:04:22PM -0700, Andrew Morton wrote:
    > On Sat, 26 Aug 2006 14:09:55 -0700 (PDT)
    > Linus Torvalds <torvalds@osdl.org> wrote:
    >
    > > I definitely want to have this fixed, and Gautham's patches look like a
    > > good thing to me, but the "trying to fix up the current mess" part was
    > > really about trying to get 2.6.18 in a mostly working state rather than
    > > anything else. I think it's been too late to try to actually _fix_ it for
    > > 2.6.18 for a long time already.
    > >
    > > So my reaction is that this redesign should go in asap after 2.6.18,
    > > unless people feel strongly that the current locking has so many bugs that
    > > people can actually _hit_ in practice that it's better to go for the
    > > redesign early.
    >
    > It certainly needs a redesign. A new sort of lock which makes it appear to
    > work won't fix races like:
    >
    > int cpufreq_update_policy(unsigned int cpu)
    > {
    > struct cpufreq_policy *data = cpufreq_cpu_get(cpu);
    >
    > ...
    >
    > lock_cpu_hotplug();
    >

    The problem with cpufreq was that it used lock_cpu_hotplug()
    in some common routines because it
    was needed in some paths and then also called the same routines
    from the CPU hotplug callback path. That is easily fixable and
    Gautham's patch 1/4 does exactly that.
    One thing I have privately suggested to Gautham is to do an audit
    of bad lock_cpu_hotplug() uses.

    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. With multiple low-level subsystems
    needing it, it also becomes difficult to work out the lock
    hierarchies. The right way to do this is what Gautham and Ingo
    are discussing - a scalable rw semaphore type lock that allows
    recursive readers.

    >
    > 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 right thing to do would be to
    do an audit and clean up the bad lock_cpu_hotplug() calls. People
    seem to have just got lazy with lock_cpu_hotplug().

    Thanks
    Dipankar
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