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

Boqun Feng
2025-07-11T13:32:24Z

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 ;-)

Regards,
Boqun

> ---
> Cheers,
> Benno
>
> > + }
> > + } else {
> > + barrier();
> > + }
> > +}