From mboxrd@z Thu Jan  1 00:00:00 1970
From: Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 0/7] preempt_count rework -v2
Date: Tue, 10 Sep 2013 14:43:06 -0700
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To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>, Andi Kleen <ak@linux.intel.com>, Peter Anvin <hpa@zytor.com>, Mike Galbraith <bitbucket@online.de>, Thomas Gleixner <tglx@linutronix.de>, Arjan van de Ven <arjan@linux.intel.com>, Frederic Weisbecker <fweisbec@gmail.com>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.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