On Mon, 6 Oct 2014, David Lang wrote:
> On Sun, 5 Oct 2014, Thomas Gleixner wrote:
>
> > >
> > > 64 bit counters on 32 bit machines are not an easy thing and could be
> >
> > Whats so hard about 64bit counters on 32bit machines?
> >
> > > expensive to handle in particular because these counters are used in
> > > performance critical hotpaths.
> >
> > The expensive overhead is a single "adcl" instruction.
>
> As I understand it, since the 64 bit math cannot be made atomic, it requires
> protecting the counter with a lock so that it can't be read while half
> updated. Aquiring a lock on every update is an expensive thing to do. It's not
> something people like to see in a fast path, especially for something of as
> low an importance as the counters.
As I said before. Both reader and writer side are already protected by
the irq descriptor lock. We take that lock anyway.
Thanks,
tglx
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