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

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Message-ID: <aHFlS96FTRgS0dH_@tardis-2.local>
Date: Fri, 11 Jul 2025 12:26:03 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Benno Lossin <lossin@...nel.org>
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 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.

Regards,
Boqun

> 
> ---
> Cheers,
> Benno