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    SubjectRe: [RFC PATCH 0/4] Refcount Based Cpu-Hotplug Revisit.
    On Tue, Oct 16, 2007 at 10:20:37AM -0700, Linus Torvalds wrote:
    > On Tue, 16 Oct 2007, Gautham R Shenoy wrote:
    >
    > Well, afaik, the patch series is fairly clean, and I'm obviously perfectly
    > happy with the approach, so I have no objections.
    >
    > But it looks buggy. This:
    >
    > +static void cpu_hotplug_begin(void)
    > +{
    > + mutex_lock(&cpu_hotplug.lock);
    > + cpu_hotplug.active_writer = current;
    > + while (cpu_hotplug.refcount) {
    > + mutex_unlock(&cpu_hotplug.lock);
    > + wait_for_completion(&cpu_hotplug.readers_done);
    > + mutex_lock(&cpu_hotplug.lock);
    > + }
    > +
    > +}
    >
    > drops the cpu_hotplug.lock, which - as far as I can see - means that
    > another process can come in and do the same, and mess up the
    > "active_writer" thing. The oerson that actually *gets* the lock may not be
    > the same one that has "active_writer" set to itself. No? Am I missing
    > something.

    Unless I am reading the patch wrongly, it seems cpu_hotplug_begin() is called
    while holding the cpu_add_remove_lock mutex. So, another CPU cannot come in
    and do the same until _cpu_down() is over.

    Thanks
    Dipankar
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