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

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Message-Id: <DB9GF0U3JJWL.3FQFMRTBO52C1@kernel.org>
Date: Fri, 11 Jul 2025 20:57:27 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Boqun Feng" <boqun.feng@...il.com>
Cc: <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>,
 <lkmm@...ts.linux.dev>, <linux-arch@...r.kernel.org>, "Miguel Ojeda"
 <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>, "Gary Guo"
 <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Andreas Hindborg" <a.hindborg@...nel.org>,
 "Alice Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>,
 "Danilo Krummrich" <dakr@...nel.org>, "Will Deacon" <will@...nel.org>,
 "Peter Zijlstra" <peterz@...radead.org>, "Mark Rutland"
 <mark.rutland@....com>, "Wedson Almeida Filho" <wedsonaf@...il.com>,
 "Viresh Kumar" <viresh.kumar@...aro.org>, "Lyude Paul" <lyude@...hat.com>,
 "Ingo Molnar" <mingo@...nel.org>, "Mitchell Levy"
 <levymitchell0@...il.com>, "Paul E. McKenney" <paulmck@...nel.org>, "Greg
 Kroah-Hartman" <gregkh@...uxfoundation.org>, "Linus Torvalds"
 <torvalds@...ux-foundation.org>, "Thomas Gleixner" <tglx@...utronix.de>,
 "Alan Stern" <stern@...land.harvard.edu>
Subject: Re: [PATCH v6 8/9] rust: sync: Add memory barriers

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