On 12/12, Srivatsa S. Bhat wrote:

On 12/12/2012 10:54 PM, Oleg Nesterov wrote:

quoted

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..

quoted

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.