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    SubjectRe: [PATCH linux-2.6 04/04] brsem: convert cpucontrol to brsem
    Nick Piggin wrote:
    > Tejun Heo wrote:
    >
    > >+/*
    > >+ * cpucontrol is a brsem used to synchronize cpu hotplug events.
    > >+ * Invoking lock_cpu_hotplug() read-locks cpucontrol and no
    > >+ * hotplugging events will occur until it's released.
    > >+ *
    > >+ * Unfortunately, brsem itself makes use of lock_cpu_hotplug() and
    > >+ * performing brsem write-lock operations on cpucontrol deadlocks.
    > >+ * This is avoided by...
    > >+ *
    > >+ * a. guaranteeing that cpu hotplug events won't occur during the
    > >+ * write-lock operations, and
    > >+ *
    > >+ * b. skipping lock_cpu_hotplug() inside brsem.
    > >+ *
    > >+ * #a is achieved by acquiring and releasing cpucontrol_mutex outside
    > >+ * cpucontrol write-lock. #b is achieved by skipping
    > >+ * lock_cpu_hotplug() inside brsem if the current task is
    > >+ * cpucontrol_mutex holder (is_cpu_hotplug_holder() test).
    > >+ *
    > >+ * Also, note that cpucontrol is first initialized with
    > >+ * BRSEM_BYPASS_INITIALIZER and then initialized again with
    > >+ * __create_brsem() instead of simply using create_brsem(). This is
    > >+ * necessary as cpucontrol brsem gets used way before brsem subsystem
    > >+ * becomes up and running.
    > >+ *
    > >+ * Until brsem is properly initialized, all brsem ops succeed
    > >+ * unconditionally. cpucontrol becomes operational only after
    > >+ * cpucontrol_init() is finished, which should be called after
    > >+ * brsem_init_early().
    > >+ */
    >
    > Mmm, this is just insane IMO.
    >
    > Note that I happen to also think the idea (brsems) have merit, and
    > that cpucontrol may be one of the places where a sane implementation
    > would actually be useful... but at least when you're introducing
    > this kind of complexity anywhere, you *really* need to be able to
    > back it up with numbers.

    The only performance-related complaint with cpu hotplug of which I'm
    aware -- that taking a cpu down on a large system can be painfully
    slow -- resides in the "write side" of the code, which is not the case
    that the brsem implementation optimizes. I think this patch would
    make that case even worse. So I don't think it's appropriate to use a
    brsem for cpu hotplug, especially without trying rwsem first.


    Nathan
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