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    Subject[PATCH 47/51] profile: 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 profile code by using this latter form of callback registration.

    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Cc: Mauro Carvalho Chehab <mchehab@redhat.com>
    Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    ---

    kernel/profile.c | 20 +++++++++++++++-----
    1 file changed, 15 insertions(+), 5 deletions(-)

    diff --git a/kernel/profile.c b/kernel/profile.c
    index 6631e1e..df27769 100644
    --- a/kernel/profile.c
    +++ b/kernel/profile.c
    @@ -591,18 +591,28 @@ out_cleanup:
    int __ref create_proc_profile(void) /* false positive from hotcpu_notifier */
    {
    struct proc_dir_entry *entry;
    + int err = 0;

    if (!prof_on)
    return 0;
    - if (create_hash_tables())
    - return -ENOMEM;
    +
    + cpu_maps_update_begin();
    +
    + if (create_hash_tables()) {
    + err = -ENOMEM;
    + goto out;
    + }
    +
    entry = proc_create("profile", S_IWUSR | S_IRUGO,
    NULL, &proc_profile_operations);
    if (!entry)
    - return 0;
    + goto out;
    proc_set_size(entry, (1 + prof_len) * sizeof(atomic_t));
    - hotcpu_notifier(profile_cpu_callback, 0);
    - return 0;
    + __hotcpu_notifier(profile_cpu_callback, 0);
    +
    +out:
    + cpu_maps_update_done();
    + return err;
    }
    module_init(create_proc_profile);
    #endif /* CONFIG_PROC_FS */


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