linux-kernel - Re: [WIP 0/3] Memory model and atomic API in Rust

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Message-ID: <Zf7tUL52AuutOSvL@Boquns-Mac-mini.home>
Date: Sat, 23 Mar 2024 07:55:12 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Andrew Lunn <andrew@...n.ch>
Cc: Kent Overstreet <kent.overstreet@...ux.dev>,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, llvm@...ts.linux.dev,
	Miguel Ojeda <ojeda@...nel.org>,	Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Andrea Parri <parri.andrea@...il.com>,	Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nicholas Piggin <npiggin@...il.com>,	David Howells <dhowells@...hat.com>,
	Jade Alglave <j.alglave@....ac.uk>,	Luc Maranget <luc.maranget@...ia.fr>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Akira Yokosawa <akiyks@...il.com>,	Daniel Lustig <dlustig@...dia.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,	kent.overstreet@...il.com,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, elver@...gle.com,
	Mark Rutland <mark.rutland@....com>,
	Thomas Gleixner <tglx@...utronix.de>,	Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Catalin Marinas <catalin.marinas@....com>,	torvalds@...ux-foundation.org,
 linux-arm-kernel@...ts.infradead.org,	linux-fsdevel@...r.kernel.org
Subject: Re: [WIP 0/3] Memory model and atomic API in Rust

On Sat, Mar 23, 2024 at 07:41:28AM -0700, Boqun Feng wrote:
> On Sat, Mar 23, 2024 at 03:29:11PM +0100, Andrew Lunn wrote:
> > > There are also issues like where one Rust thread does a store(..,
> > > RELEASE), and a C thread does a rcu_deference(), in practice, it
> > > probably works but no one works out (and no one would work out) a model
> > > to describe such an interaction.
> > 
> > Isn't that what Paul E. McKenney litmus tests are all about?
> > 
> 
> Litmus tests (or herd, or any other memory model tools) works for either
> LKMM or C++ memory model. But there is no model I'm aware of works for
> the communication between two memory models. So for example:
> 
> 	Rust thread:
> 
> 	let mut foo: Box<Foo> = ...;
> 	foo.a = 1;
> 	let global_ptr: &AtomicPtr = ...;
> 	global_ptr.store(foo.leak() as _, RELEASE);
> 
> 	
> 	C thread:
> 
> 	rcu_read_lock();
> 
> 	foo = rcu_dereference(global_ptr);
> 	if (foo) {
> 		r1 = foo->a;
> 	}
> 	
> 	rcu_read_unlock();
> 
> no tool or model yet to guarantee "r1" is 1, but yeah, in practice for
> the case we care, it's probably guaranteed. But no tool or model means
> challenging for code reasoning.
> 

There are also cases where two similar APIs from C++ memory model and
LKMM have different semantics, for example, a SeqCst atomic in C++
memory model doesn't imply a full barrier, while a fully ordered LKMM
atomic does:

	Rust:

	a.store(1, RELAXED);
	x.fetch_add(1, SeqCst);
	b.store(2, RELAXED);

	// ^ writes to a and b are not ordered.

	C:

	WRITE_ONCE(*a, 1);
	atomic_fetch_add(x, 1);
	WRITE_ONCE(*b, 2);

	// ^ writes to a and b are ordered.

So if you used to have two parts synchronizing each other with LKMM
atomics, converting one side to Rust *and* using Rust atomics requires
much caution.

Regards,
Boqun

> Regards,
> Boqun
> 
> > tools/memory-model/litmus-test
> > 
> > 	Andrew