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    Subject[PATCH 05/51] ia64, palinfo: Fix CPU hotplug callback registration
    Date
    Subsystems that want to register CPU hotplug callbacks, as well as perform
    initialization for the CPUs that are already online, often do it as shown
    below:

    get_online_cpus();

    for_each_online_cpu(cpu)
    init_cpu(cpu);

    register_cpu_notifier(&foobar_cpu_notifier);

    put_online_cpus();

    This is wrong, since it is prone to ABBA deadlocks involving the
    cpu_add_remove_lock and the cpu_hotplug.lock (when running concurrently
    with CPU hotplug operations).

    Instead, the correct and race-free way of performing the callback
    registration is:

    cpu_maps_update_begin();

    for_each_online_cpu(cpu)
    init_cpu(cpu);

    /* Note the use of the double underscored version of the API */
    __register_cpu_notifier(&foobar_cpu_notifier);

    cpu_maps_update_done();


    Fix the palinfo code in ia64 by using this latter form of callback
    registration.

    Cc: Tony Luck <tony.luck@intel.com>
    Cc: Fenghua Yu <fenghua.yu@intel.com>
    Cc: linux-ia64@vger.kernel.org
    Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    ---

    arch/ia64/kernel/palinfo.c | 6 +++++-
    1 file changed, 5 insertions(+), 1 deletion(-)

    diff --git a/arch/ia64/kernel/palinfo.c b/arch/ia64/kernel/palinfo.c
    index ab33328..cfaa06f 100644
    --- a/arch/ia64/kernel/palinfo.c
    +++ b/arch/ia64/kernel/palinfo.c
    @@ -996,13 +996,17 @@ palinfo_init(void)
    if (!palinfo_dir)
    return -ENOMEM;

    + cpu_maps_update_begin();
    +
    /* Create palinfo dirs in /proc for all online cpus */
    for_each_online_cpu(i) {
    create_palinfo_proc_entries(i);
    }

    /* Register for future delivery via notify registration */
    - register_hotcpu_notifier(&palinfo_cpu_notifier);
    + __register_hotcpu_notifier(&palinfo_cpu_notifier);
    +
    + cpu_maps_update_done();

    return 0;
    }


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