Messages in this thread | | | Date | Tue, 1 Oct 2013 17:00:30 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] hotplug: Optimize {get,put}_online_cpus() |
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On 10/01, Paul E. McKenney wrote: > > On Tue, Oct 01, 2013 at 04:14:29PM +0200, Oleg Nesterov wrote: > > > > But please note another email, it seems to me we can simply kill > > cpuhp_seq and all the barriers in cpuhp_readers_active_check(). > > If you don't have cpuhp_seq, you need some other way to avoid > counter overflow.
I don't think so. Overflows (espicially "unsigned") should be fine and in fact we can't avoid them.
Say, a task does get() on CPU_0 and put() on CPU_1, after that we have
CTR[0] == 1, CTR[1] = (unsigned)-1
iow, the counter was already overflowed (underflowed). But this is fine, all we care about is CTR[0] + CTR[1] == 0, and this is only true because of another overflow.
But probably you meant another thing,
> Which might be provided by limited number of > tasks, or, on 64-bit systems, 64-bit counters.
perhaps you meant that max_threads * max_depth can overflow the counter? I don't think so... but OK, perhaps this counter should be u_long.
But how cpuhp_seq can help?
Oleg.
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