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    Subject[PATCH 23/51] x86, amd, uncore: 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 amd-uncore code in x86 by using this latter form of callback
    registration.

    Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Paul Mackerras <paulus@samba.org>
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: "H. Peter Anvin" <hpa@zytor.com>
    Cc: x86@kernel.org
    Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    ---

    arch/x86/kernel/cpu/perf_event_amd_uncore.c | 7 ++++---
    1 file changed, 4 insertions(+), 3 deletions(-)

    diff --git a/arch/x86/kernel/cpu/perf_event_amd_uncore.c b/arch/x86/kernel/cpu/perf_event_amd_uncore.c
    index 754291a..3982ef0 100644
    --- a/arch/x86/kernel/cpu/perf_event_amd_uncore.c
    +++ b/arch/x86/kernel/cpu/perf_event_amd_uncore.c
    @@ -531,15 +531,16 @@ static int __init amd_uncore_init(void)
    if (ret)
    return -ENODEV;

    - get_online_cpus();
    + cpu_maps_update_begin();
    +
    /* init cpus already online before registering for hotplug notifier */
    for_each_online_cpu(cpu) {
    amd_uncore_cpu_up_prepare(cpu);
    smp_call_function_single(cpu, init_cpu_already_online, NULL, 1);
    }

    - register_cpu_notifier(&amd_uncore_cpu_notifier_block);
    - put_online_cpus();
    + __register_cpu_notifier(&amd_uncore_cpu_notifier_block);
    + cpu_maps_update_done();

    return 0;
    }


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