Re: [PATCH v6 8/9] rust: sync: Add memory barriers

Boqun Feng
2025-07-11T19:26:07Z

On Fri, Jul 11, 2025 at 08:57:27PM +0200, Benno Lossin wrote:
> On Fri Jul 11, 2025 at 3:32 PM CEST, Boqun Feng wrote:
> > On Fri, Jul 11, 2025 at 10:57:48AM +0200, Benno Lossin wrote:
> > [...]
> >> > +}
> >> > +
> >> > +/// A full memory barrier.
> >> > +///
> >> > +/// A barrier that prevents compiler and CPU from reordering memory accesses across the barrier.
> >> > +pub fn smp_mb() {
> >> > + if cfg!(CONFIG_SMP) {
> >> > + // SAFETY: `smp_mb()` is safe to call.
> >> > + unsafe {
> >> > + bindings::smp_mb();
> >>
> >> Does this really work? How does the Rust compiler know this is a memory
> >> barrier?
> >>
> >
> > - Without INLINE_HELPER, this is an FFI call, it's safe to assume that
> > Rust compiler would treat it as a compiler barrier and in smp_mb() a
> > real memory barrier instruction will be executed.
> >
> > - With INLINE_HELPER, this will be inlined as an asm block with "memory"
> > as clobber, and LLVM will know it's a compiler memory barrier, and the
> > real memory barrier instruction guarantees it's a memory barrier at
> > CPU reordering level as well.
> >
> > Think about this, SpinLock and Mutex need memory barriers for critical
> > section, if this doesn't work, then SpinLock and Mutex don't work
> > either, then we have a bigger problem ;-)
>
> By "this not working" I meant that he barrier would be too strong :)
>
> So essentially without INLINE_HELPER, all barriers in this file are the
> same, but with it, we get less strict ones?

Not the same, each barrier function may generate a different hardware
instruction ;-)

I would say for a Rust function (e.g. smp_mb()), the difference between
with and without INLINE_HELPER is:

- with INLINE_HELPER enabled, they behave exactly like a C function
calling a C smp_mb().

- without INLINE_HELPER enabled, they behave like a C function calling
a function that never inlined:

void outofline_smp_mb(void)
{
smp_mb();
}

It might be stronger than the "with INLINE_HELPER" case but both are
correct regarding memory ordering.

Regards,
Boqun

>
> ---
> Cheers,
> Benno