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

[Oleg Nesterov]

On 12/12, Oleg Nesterov wrote:
>
> On 12/11, Srivatsa S. Bhat wrote:
> >
> > IOW, the hotplug readers just increment/decrement their per-cpu refcounts
> > when no writer is active.
>
> plus cli/sti ;) and increment/decrement are atomic.
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

OOPS, sorry I was going to say "adds mb()".

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().

At the same time reader_nested_percpu() can be checked before mb().

Oleg.

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