Re: [RFC] LKMM: Add volatile_if()

Paul E. McKenney
2021-06-06T00:14:21Z

On Sat, Jun 05, 2021 at 10:57:39AM -0400, Alan Stern wrote:
> 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.

That does sound good, but...

Current compilers beg to differ at -O2: https://godbolt.org/z/5K55Gardn

------------------------------------------------------------------------
#define READ_ONCE(x) (*(volatile typeof(x) *)&(x))
#define WRITE_ONCE(x, val) (READ_ONCE(x) = (val))
#define barrier() __asm__ __volatile__("": : :"memory")

int x, y;

int main(int argc, char *argv[])
{
if (READ_ONCE(x)) {
barrier();
WRITE_ONCE(y, 1);
} else {
barrier();
WRITE_ONCE(y, 1);
}
return 0;
}
------------------------------------------------------------------------

Both gcc and clang generate a load followed by a store, with no branch.
ARM gets the same results from both compilers.

As Linus suggested, removing one (but not both!) invocations of barrier()
does cause a branch to be emitted, so maybe that is a way forward.
Assuming it is more than just dumb luck, anyway. :-/

Thanx, Paul