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Sender: Tejun Heo <htejun@gmail.com>
Date: Mon, 24 Jun 2013 10:55:35 -0700
From: Tejun Heo <tj@kernel.org>
To: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
Subject: Re: [PATCH 22/45] percpu_counter: Use get/put_online_cpus_atomic()
 to prevent CPU offline
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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@zeniv.linux.org.uk>
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
...
> @@ -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