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    Subject[PATCH v2 51/52] net/core/flow.c: 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 code in net/core/flow.c by using this latter form of callback
    registration.

    Cc: Li RongQing <roy.qing.li@gmail.com>
    Cc: Sasha Levin <sasha.levin@oracle.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Chris Metcalf <cmetcalf@tilera.com>
    Cc: Ingo Molnar <mingo@kernel.org>
    Cc: netdev@vger.kernel.org
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    ---

    net/core/flow.c | 8 +++++++-
    1 file changed, 7 insertions(+), 1 deletion(-)

    diff --git a/net/core/flow.c b/net/core/flow.c
    index dfa602c..9a2151f 100644
    --- a/net/core/flow.c
    +++ b/net/core/flow.c
    @@ -456,6 +456,8 @@ static int __init flow_cache_init(struct flow_cache *fc)
    if (!fc->percpu)
    return -ENOMEM;

    + cpu_notifier_register_begin();
    +
    for_each_online_cpu(i) {
    if (flow_cache_cpu_prepare(fc, i))
    goto err;
    @@ -463,7 +465,9 @@ static int __init flow_cache_init(struct flow_cache *fc)
    fc->hotcpu_notifier = (struct notifier_block){
    .notifier_call = flow_cache_cpu,
    };
    - register_hotcpu_notifier(&fc->hotcpu_notifier);
    + __register_hotcpu_notifier(&fc->hotcpu_notifier);
    +
    + cpu_notifier_register_done();

    setup_timer(&fc->rnd_timer, flow_cache_new_hashrnd,
    (unsigned long) fc);
    @@ -479,6 +483,8 @@ err:
    fcp->hash_table = NULL;
    }

    + cpu_notifier_register_done();
    +
    free_percpu(fc->percpu);
    fc->percpu = NULL;



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