On Sun, Jun 23, 2013 at 07:12:59PM +0530, Srivatsa S. Bhat wrote:
> Once stop_machine() is gone from the CPU offline path, we won't be able
> to depend on disabling preemption to prevent CPUs from going offline
> from under us.
>
> Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
> offline, while invoking from atomic context.
>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
...
> @@ -98,6 +98,7 @@ s64 __percpu_counter_sum(struct percpu_counter *fbc)
> s64 ret;
> int cpu;
>
> + get_online_cpus_atomic();
> raw_spin_lock(&fbc->lock);
> ret = fbc->count;
> for_each_online_cpu(cpu) {
> @@ -105,6 +106,7 @@ s64 __percpu_counter_sum(struct percpu_counter *fbc)
> ret += *pcount;
> }
> raw_spin_unlock(&fbc->lock);
> + put_online_cpus_atomic();
I don't think this is necessary. CPU on/offlining is explicitly
handled via the hotplug callback which synchronizes through fbc->lock.
__percpu_counter_sum() racing with actual on/offlining doesn't affect
correctness and adding superflous get_online_cpus_atomic() around it
can be misleading.
Thanks.
--
tejun
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