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

Boqun Feng
2025-07-11T21:34:59Z

On Fri, Jul 11, 2025 at 11:04:09PM +0200, Benno Lossin wrote:
> On Fri Jul 11, 2025 at 9:26 PM CEST, Boqun Feng wrote:
> > 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.
>
> Yes, this is exactly what I meant with "not working" :)
>

But be stronger is still "working" ;-)

BTW, replying you made me realize I should make these function
#[inline(always)] so thank you ;-)

Regards,
Boqun

> ---
> Cheers,
> Benno
>