Re: [RFC PATCH 01/20] asm-generic/mmiowb: Add generic implementation of mmiowb() tracking

[Linus Torvalds]

On Fri, Feb 22, 2019 at 1:49 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> The case we want to go fast is the spin-lock and unlock case, not the
> "set pending" case.
>
> And the way you implemented this, it's exactly the wrong way around.

Oh, one more comment: couldn't we make that mmiowb flag be right next
to the preemption count?

Because that's the common case anyway, where a spinlock increments the
preemption count too. If we put the mmiowb state in the same
cacheline, we don't cause extra cache effects, which is what really
matters, I guess.

I realize this is somewhat inconvenient, because some architectures
put preempt count in the thread structure, and others do it as a
percpu variable. But maybe the architecture could just declare where
the mmiowb state is?

                 Linus