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

Benno Lossin
2025-07-11T21:04:16Z

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" :)

---
Cheers,
Benno