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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <DB9J417F3XRT.1XGPA6VLF9T8K@kernel.org>
Date: Fri, 11 Jul 2025 23:04:09 +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 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