Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPUoffline from atomic context

[Oleg Nesterov]

On 12/12, Srivatsa S. Bhat wrote:
>
> On 12/12/2012 10:54 PM, Oleg Nesterov wrote:
>
> > And when I look at get_online_cpus_atomic() again it uses rmb(). This
> > doesn't look correct, we need the full barrier between this_cpu_inc()
> > and writer_active().
>
> Hmm..
>
> > At the same time reader_nested_percpu() can be checked before mb().
>
> I thought that since the increment and the check (reader_nested_percpu)
> act on the same memory location, they will naturally be run in the given
> order, without any need for barriers. Am I wrong?

And this is what I meant, you do not need a barrier before
reader_nested_percpu().

But you need to ensure that WRITE(reader_percpu_refcnt) and READ(writer_signal)
can't be reordered, so you need mb() in between. rmb() can serialize LOADs and
STOREs.

Or I misunderstood?

Oleg.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/