linux-kernel - Re: [PATCH RFC 0/2] rcu box container for Rust + maple tree load

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Message-ID: <aXDRHerRiM7TzACy@tardis-2.local>
Date: Wed, 21 Jan 2026 21:14:05 +0800
From: Boqun Feng <boqun.feng@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: "Paul E. McKenney" <paulmck@...nel.org>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Gary Guo <gary@...yguo.net>, Miguel Ojeda <ojeda@...nel.org>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Josh Triplett <josh@...htriplett.org>,
	Uladzislau Rezki <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang@...ux.dev>,
	Andrew Ballance <andrewjballance@...il.com>,
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
	rcu@...r.kernel.org, maple-tree@...ts.infradead.org,
	linux-mm@...ck.org
Subject: Re: [PATCH RFC 0/2] rcu box container for Rust + maple tree load_rcu

On Wed, Jan 21, 2026 at 12:10:54PM +0000, Alice Ryhl wrote:
> On Sat, Jan 17, 2026 at 10:00:19PM +0800, Boqun Feng wrote:
> > On Sat, Jan 17, 2026 at 01:12:08PM +0000, Alice Ryhl wrote:
> > [...]
> > > > > 1) "relaxed atomic" does not sound like something that provides an
> > > > >    address dependency to me.
> > > > 
> > > > If you look at rcu_dereference(), it's a READ_ONCE(), which is the same
> > > > as a relaxed atomic load, and yes in LKMM, relaxed atomic load provides
> > > > address dependency (Please see the DEPENDENCY part in
> > > > tools/memory-model/Documentation/explanation.txt).
> > > 
> > > You argued that we should rename READ_ONCE() to atomic load on that
> > > other patch series because "atomic load" naming is better than what LKMM
> > > normally uses. Fine, but relaxed atomic load is a much worse name than
> > 
> > To be clear, in that series, my argument was not about naming, it's
> > about READ_ONCE() being more powerful than atomic load (no, not because
> > of address dependency, they are the same on that, it's because of the
> > behaviors of them regarding a current access on the same memory
> > location), and we want user to specify the intention more clearly.
> 
> Expressing intent more clearly is fine with me. I still think it's weird
> for us to not have READ_ONCE() when it's a primitive operation of our
> memory model, though.
> 

But in our memory model, it's exact the same as atomic_read() (see
tools/memory-model/linux-kernel.def and search for "atomic_read"), so
why do we want to have both? ;-)

> And I also think we should consider using an implementation along the
> lines of what I shared for our atomic_load() or READ_ONCE() or whatever
> you wish to call it. The perf impact of helpers makes me sad.
> 

I'm not totally against that, it'll actually help Atomic as well, I also
hope that we can use `asm!()` to implement the cases where
`{read,write}_volatile()` cannot cover. However currently I would rely
on helper inlining to resolve this to avoid duplicate implementations.

> > > READ_ONCE() if what you want to convey is "has address dependency".
> > > That's not what "relaxed" means!
> > > 
> > 
> > Also note that my previous reply was explaining why we don't need to
> > call rcu_dereference() from Rust, because implementation-wise the
> > LKMM relaxed atomic load provides the address dependency. Depending on
> > what we want to do, we can limit this address dependency only to
> > rcu_dereference() and make it a special case, this means we disallow the
> > address dependency provided by the "relaxed" in normal cases. Or we can
> > add a Consume ordering (a type alias to Relaxed) that makes user to
> > explicitly use it when they rely on the address dependency. I think
> > either would resolve your concern about the name of "relaxed".
> 
> Yes, I suppose that would resolve my concern about the name.
> 
> Well, I do have one partial concern there, which is that the wrong name
> will show up in stack traces as long as helpers are used.
> 

I think it won't be a problem since in LKMM, atomic_read() and
READ_ONCE() are identical.

Regards,
Boqun

> > > I suppose you can argue that the word "relaxed" means different things
> > > in LKMM than it does elsewhere, but I looked over the doc you mentioned,
> > > and there the LKMM calls said operation READ_ONCE(). The word "relaxed"
> > > does not appear even once. If we're going to change terminology / use
> > > new terminology, let's at least pick terminology that's not
> > > contradictory with the rest of the world.
> > > 
[...]