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    Subject[PATCH 27/51] x86, kvm: 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 kvm code in x86 by using this latter form of callback registration.

    Cc: Gleb Natapov <gleb@kernel.org>
    Cc: Paolo Bonzini <pbonzini@redhat.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: "H. Peter Anvin" <hpa@zytor.com>
    Cc: x86@kernel.org
    Cc: kvm@vger.kernel.org
    Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    ---

    arch/x86/kvm/x86.c | 7 ++++++-
    1 file changed, 6 insertions(+), 1 deletion(-)

    diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
    index 39c28f09..e3893b7 100644
    --- a/arch/x86/kvm/x86.c
    +++ b/arch/x86/kvm/x86.c
    @@ -5365,7 +5365,8 @@ static void kvm_timer_init(void)
    int cpu;

    max_tsc_khz = tsc_khz;
    - register_hotcpu_notifier(&kvmclock_cpu_notifier_block);
    +
    + cpu_maps_update_begin();
    if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) {
    #ifdef CONFIG_CPU_FREQ
    struct cpufreq_policy policy;
    @@ -5382,6 +5383,10 @@ static void kvm_timer_init(void)
    pr_debug("kvm: max_tsc_khz = %ld\n", max_tsc_khz);
    for_each_online_cpu(cpu)
    smp_call_function_single(cpu, tsc_khz_changed, NULL, 1);
    +
    + __register_hotcpu_notifier(&kvmclock_cpu_notifier_block);
    + cpu_maps_update_done();
    +
    }

    static DEFINE_PER_CPU(struct kvm_vcpu *, current_vcpu);


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