Re: [RFC] LKMM: Add volatile_if()

Alan Stern
2021-06-05T14:57:42Z

On Fri, Jun 04, 2021 at 03:19:11PM -0700, Linus Torvalds wrote:
> Now, part of this is that I do think that in *general* we should never
> use this very suble load-cond-store pattern to begin with. We should
> strive to use more smp_load_acquire() and smp_store_release() if we
> care about ordering of accesses. They are typically cheap enough, and
> if there's much of an ordering issue, they are the right things to do.
>
> I think the whole "load-to-store ordering" subtle non-ordered case is
> for very very special cases, when you literally don't have a general
> memory ordering, you just have an ordering for *one* very particular
> access. Like some of the very magical code in the rw-semaphore case,
> or that smp_cond_load_acquire().
>
> IOW, I would expect that we have a handful of uses of this thing. And
> none of them have that "the conditional store is the same on both
> sides" pattern, afaik.
>
> And immediately when the conditional store is different, you end up
> having a dependency on it that orders it.
>
> But I guess I can accept the above made-up example as an "argument",
> even though I feel it is entirely irrelevant to the actual issues and
> uses we have.

Indeed, the expansion of the currently proposed version of

volatile_if (A) {
B;
} else {
C;
}

is basically the same as

if (A) {
barrier();
B;
} else {
barrier();
C;
}

which is just about as easy to write by hand. (For some reason my
fingers don't like typing "volatile_"; the letters tend to get
scrambled.)

So given that:

1. Reliance on control dependencies is uncommon in the kernel,
and

2. The loads in A could just be replaced with load_acquires
at a low penalty (or store-releases could go into B and C),

it seems that we may not need volatile_if at all! The only real reason
for having it in the first place was to avoid the penalty of
load-acquire on architectures where it has a significant cost, when the
control dependency would provide the necessary ordering for free. Such
architectures are getting less and less common.

Alan