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

[Boqun Feng]

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