Re: [RFC] LKMM: Add volatile_if()

Segher Boessenkool
2021-06-06T19:57:09Z

On Sun, Jun 06, 2021 at 11:48:32AM -0700, Linus Torvalds wrote:
> On Sun, Jun 6, 2021 at 11:43 AM Segher Boessenkool
> <segher@kernel.crashing.org> wrote:
> >
> > You truly should have written a branch in tthe asm if you truly wanted
> > a branch instruction.
>
> That's exactly what I don't want to do, and what the original patch by
> PeterZ did.

Yes, I know. But it is literally the *only* way to *always* get a
conditional branch: by writing one.

> And to work well, it needs "asm goto", which is so recent that a lot
> of compilers don't support it (thank God for clang dragging gcc
> kicking and screaming to implement it at all - I'd asked for it over a
> decade ago).

GCC has had it since 2009.

> So you get bad code generation in a lot of cases, which entirely
> obviates the _point_ of this all - which is that we can avoid an
> expensive operation (a memory barrier) by just doing clever code
> generation.
>
> So if we can't get the clever code generation, it's all pretty much
> moot, imnsho.

Yes.

Segher