Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

[Peter Zijlstra]

On Wed, Oct 02, 2013 at 04:00:20PM +0200, Oleg Nesterov wrote:
> And again, even
> 
> 	for (;;) {
> 		percpu_down_write();
> 		percpu_up_write();
> 	}
> 
> should not completely block the readers.

Sure there's a tiny window, but don't forget that a reader will have to
wait for the gp_state cacheline to transfer to shared state and the
per-cpu refcount cachelines to be brought back into exclusive mode and
the above can be aggressive enough that by that time we'll observe
state == blocked again.

So I don't think that in practise a reader will get in.

Also, since the write side is exposed to userspace; you've got an
effective DoS.

So I'll stick to waitcount -- as you can see in the patches I've just
posted.
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