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

[Will Deacon]

On Fri, Feb 22, 2019 at 01:55:20PM -0800, Linus Torvalds wrote:
> 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?

I think that should be doable... I'll have a play.

Will