Re: [PATCH 0/7] preempt_count rework -v2

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, Sep 10, 2013 at 2:25 PM, Peter Zijlstra <peterz@infradead.org> wrote:
>
> Here's one that builds and boots on kvm until wanting to mount root.
>
> I'm not entirely sure on the "ir" vs "er" thing and atomic64_t and
> local_t are inconsistent wrt that so I'm too.

"i" is "any constant", while "e" is "32-bit signed constant".

And I think all of the 64-bit ones should probably be "e", because
afaik there is no way to add a 64-bit constant directly to memory (you
have to load it into a register first).

Of course, in reality, the constant is always just 1 or -1 or
something like that, so nobody will ever notice the incorrect case...

And it doesn't matter for the 32-bit cases, obviously, but we could
just make them all be "e" for simplicity.

That said, looking at your patch, I get the *very* strong feeling that
we could make a macro that does all the repetitions for us, and then
have a

  GENERATE_RMW(atomic_sub_and_test, LOCK_PREFIX "subl", "e", "")
  GENERATE_RMW(atomic_dec_and_test, LOCK_PREFIX "decl", "e", "")
  ..
  GENERATE_RMW(atomic_add_negative, LOCK_PREFIX "addl", "s", "")

  GENERATE_RMW(local_sub_and_test, "subl", "e", __percpu_prefix)
  ...

etc.

I'm sure the macro would be nasty as hell (and I bet it needs a few
more arguments), but then we'd avoid the repetition..

                Linus