linux-kernel - Re: C aggregate passing (Rust kernel policy)

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Message-ID: <2949f6dc-51fe-4a28-b44d-5a38796303e8@ralfj.de>
Date: Fri, 28 Feb 2025 08:53:11 +0100
From: Ralf Jung <post@...fj.de>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Kent Overstreet <kent.overstreet@...ux.dev>,
 Martin Uecker <uecker@...raz.at>, "Paul E. McKenney" <paulmck@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>, Ventura Jack <venturajack85@...il.com>,
 Gary Guo <gary@...yguo.net>, airlied@...il.com, boqun.feng@...il.com,
 david.laight.linux@...il.com, ej@...i.de, gregkh@...uxfoundation.org,
 hch@...radead.org, hpa@...or.com, ksummit@...ts.linux.dev,
 linux-kernel@...r.kernel.org, miguel.ojeda.sandonis@...il.com,
 rust-for-linux@...r.kernel.org
Subject: Re: C aggregate passing (Rust kernel policy)

Hi,

On 27.02.25 20:15, Linus Torvalds wrote:
> On Thu, 27 Feb 2025 at 10:33, Ralf Jung <post@...fj.de> wrote:
>>
>> The way you do global flags in Rust is like this:
> 
> Note that I was really talking mainly about the unsafe cases, an din
> particular when interfacing with C code.

When Rust code and C code share memory that is concurrently accessed, all 
accesses to that from the Rust side must be explicitly marked as atomic. A 
pointer to such a memory should look like `&AtomicBool` in Rust, not `*mut 
bool`. To my knowledge, the kernel already has appropriate APIs for that. That 
will then ensure things behave like the AtomicBool example.

Kind regards,
Ralf

> 
> Also, honestly:
> 
>> FLAG.store(true, Ordering::SeqCst); // or release/acquire/relaxed
> 
> I suspect in reality it would be hidden as accessor functions, or
> people just continue to write things in C.
> 
> Yes, I know all about the C++ memory ordering. It's not only a
> standards mess, it's all very illegible code too.
> 
>               Linus