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    Subject[PATCH 03/51] CPU hotplug, perf: 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 perf subsystem's hotplug notifier by using this latter form of
    callback registration.

    Also provide a bare-bones version of perf_cpu_notifier() that doesn't
    invoke the notifiers for the already online CPUs. This would be useful
    for subsystems that need to perform a different set of initialization
    for the already online CPUs, or don't need the initialization altogether.

    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>
    Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    ---

    include/linux/perf_event.h | 16 +++++++++++++++-
    1 file changed, 15 insertions(+), 1 deletion(-)

    diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
    index e56b07f..f4057da 100644
    --- a/include/linux/perf_event.h
    +++ b/include/linux/perf_event.h
    @@ -835,6 +835,8 @@ do { \
    { .notifier_call = fn, .priority = CPU_PRI_PERF }; \
    unsigned long cpu = smp_processor_id(); \
    unsigned long flags; \
    + \
    + cpu_maps_update_begin(); \
    fn(&fn##_nb, (unsigned long)CPU_UP_PREPARE, \
    (void *)(unsigned long)cpu); \
    local_irq_save(flags); \
    @@ -843,9 +845,21 @@ do { \
    local_irq_restore(flags); \
    fn(&fn##_nb, (unsigned long)CPU_ONLINE, \
    (void *)(unsigned long)cpu); \
    - register_cpu_notifier(&fn##_nb); \
    + __register_cpu_notifier(&fn##_nb); \
    + cpu_maps_update_done(); \
    } while (0)

    +/*
    + * Bare-bones version of perf_cpu_notifier(), which doesn't invoke the
    + * callback for already online CPUs.
    + */
    +#define __perf_cpu_notifier(fn) \
    +do { \
    + static struct notifier_block fn##_nb = \
    + { .notifier_call = fn, .priority = CPU_PRI_PERF }; \
    + \
    + __register_cpu_notifier(&fn##_nb); \
    +} while (0)

    struct perf_pmu_events_attr {
    struct device_attribute attr;


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