On Wed, Jan 4, 2012 at 9:00 AM, Christoph Lameter [off-list ref] wrote:

xadd is 3 cycles. add is one cycle.

On some uarchs. On new uarchs it can be a single cycle, I think, and
on some uarchs it will even be microcoded and/or only go in one pipe
because it has that odd behavior that it writes both to memory and a
register, and thus doesn't fit the normal fastpaths.

The point is, xadd isn't actually any faster than just doing the
regular "add and read". It's *slower*.

There really isn't ever any reason to use xadd on percpu variables.
That's my point. You claimed that there was a performance advantage.
There really isn't.

So why are you still arguing?

                           Linus