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

Benno Lossin
2025-07-11T18:57:44Z

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?

---
Cheers,
Benno