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    Subject[PATCH v2 36/52] zsmalloc: 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_notifier_register_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_notifier_register_done();


    Fix the zsmalloc code by using this latter form of callback registration.

    Cc: Minchan Kim <minchan@kernel.org>
    Cc: Nitin Gupta <ngupta@vflare.org>
    Cc: Ingo Molnar <mingo@kernel.org>
    Cc: linux-mm@kvack.org
    Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    ---

    mm/zsmalloc.c | 17 ++++++++++++++---
    1 file changed, 14 insertions(+), 3 deletions(-)

    diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
    index c03ca5e..36b4591 100644
    --- a/mm/zsmalloc.c
    +++ b/mm/zsmalloc.c
    @@ -814,21 +814,32 @@ static void zs_exit(void)
    {
    int cpu;

    + cpu_notifier_register_begin();
    +
    for_each_online_cpu(cpu)
    zs_cpu_notifier(NULL, CPU_DEAD, (void *)(long)cpu);
    - unregister_cpu_notifier(&zs_cpu_nb);
    + __unregister_cpu_notifier(&zs_cpu_nb);
    +
    + cpu_notifier_register_done();
    }

    static int zs_init(void)
    {
    int cpu, ret;

    - register_cpu_notifier(&zs_cpu_nb);
    + cpu_notifier_register_begin();
    +
    + __register_cpu_notifier(&zs_cpu_nb);
    for_each_online_cpu(cpu) {
    ret = zs_cpu_notifier(NULL, CPU_UP_PREPARE, (void *)(long)cpu);
    - if (notifier_to_errno(ret))
    + if (notifier_to_errno(ret)) {
    + cpu_notifier_register_done();
    goto fail;
    + }
    }
    +
    + cpu_notifier_register_done();
    +
    return 0;
    fail:
    zs_exit();


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