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    SubjectRe: [RFC][PATCH 4/4] Rename lock_cpu_hotplug/unlock_cpu_hotplug
    On Thu, Aug 24, 2006 at 05:00:26PM +0200, Ingo Molnar wrote:
    >
    > * Nick Piggin <nickpiggin@yahoo.com.au> wrote:
    >
    > > It really is just like a reentrant rw semaphore... I don't see the
    > > point of the name change, but I guess we don't like reentrant locks so
    > > calling it something else might go down better with Linus ;)
    >
    > what would fit best is a per-cpu scalable (on the read-side)
    > self-reentrant rw mutex. We are doing cpu hotplug locking in things like
    > fork or the slab code, while most boxes will do a CPU hotplug event only
    > once in the kernel's lifetime (during bootup), so a classic global
    > read-write lock is unjustified.

    I agree. However, I was not sure if anything else other than for cpu_hotplug,
    needs a self-reentrent rwmutex in the kernel.
    Which is why I did not expose the locking(at least the write side of it)
    outside. We don't want too many lazy programmers anyway!

    However, even in case of cpu_hotplug, if we want to prevent
    a hotplug event in some critical region where we are not going to sleep,
    we may as well use preempt_disable[/enable]. Because __stop_machine_run waits
    for all the tasks in the fast-path to complete before it changes
    the online_cpus map, if I am not mistaken.

    Only when you want your local online cpu_map to remain intact when
    you wake up from sleep, should you use cpu_hotplug *lock*.

    Ingo?

    --
    Gautham R Shenoy
    Linux Technology Center
    IBM India.
    "Freedom comes with a price tag of responsibility, which is still a bargain,
    because Freedom is priceless!"
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