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    SubjectRe: [PATCH 00/51] CPU hotplug: Fix issues with callback registration
    Hi,

    On Thu, Feb 06, 2014 at 03:34:36AM +0530, Srivatsa S. Bhat wrote:
    > Hi,
    >
    >
    > To solve these issues and provide a race-free method to register CPU hotplug
    > callbacks, this patchset introduces new variants of the callback registration
    > APIs that don't hold the cpu_add_remove_lock, and exports the
    > cpu_add_remove_lock via cpu_maps_update_begin/done() for use by various
    > subsystems. With this in place, the following code snippet will register a
    > hotplug callback as well as initialize already online CPUs without any race
    > conditions.
    >
    > cpu_maps_update_begin();
    >
    > for_each_online_cpu(cpu)
    > init_cpu(cpu);
    >
    > /* This doesn't take the cpu_add_remove_lock */
    > __register_cpu_notifier(&foobar_cpu_notifier);
    >
    > cpu_maps_update_done();
    >

    Couple of comments:

    Right now, cpu_add_remove_lock is being used to
    1) Serialize the cpu-hotplug writers.

    2) Serialize accesses to cpu_present/possible_map.

    3) Serialize updates to the cpu_chain (the cpu hotplug notifier chain).

    - This is necessary to ensure that registration of notifiers and
    invocation of CPU_POST_DEAD notifications don't race with each
    other. Else we could have used get/put_online_cpus() in
    register_cpu_notifier() and this patch series wouldn't have been
    necessary.

    4) Bulk cpu-hotplug (disable/enable_non_boot_cpus), but this is a
    special case of 1).

    CPU_POST_DEAD notification, is invoked with the cpu_hotplug.lock
    dropped. This was necessary for subsystems which would be waiting for
    some other thread to finish some work, and that other thread could
    invoke get_online_cpus(). If CPU_POST_DEAD notification were issued
    without dropping the cpu_hotplug.lock, this would lead to a deadlock
    as the notifier would be left stuck waiting for the thread which is
    blocked in get_online_cpus().

    It was introduced to ensure that multithreaded workqueues can safely
    use get_online_cpus() [https://lkml.org/lkml/2008/6/29/121].

    As of now, only two subsystems use this notification and workqueues is
    _not_ one of them!
    * arch/x86/kernel/cpu/mcheck/mce.c:mce_cpu_callback()
    * drivers/cpufreq/cpufreq.c:cpufreq_cpu_callback()
    I haven't yet audited these two cases to see if they really need this
    to be handled in CPU_POST_DEAD or if they can be handled in CPU_DEAD.

    Also can we have an alternate API, something like
    cpu_hotplug_register_begin/end() instead of reusing
    cpu_maps_update_begin/end() for this usage, since in most of the
    patches that follow, we're not touching the any of the cpu_*_maps!

    >
    >
    > Regards,
    > Srivatsa S. Bhat
    > IBM Linux Technology Center

    --
    Thanks and Regards
    gautham.



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