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

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Message-ID: <aWuV858wU3MeYeaX@tardis-2.local>
Date: Sat, 17 Jan 2026 22:00:19 +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 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.

> 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".

> 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.
> 
> > > 2) How do you intend to provide mutable access? By waiting a grace
> > >    period?
> > 
> > Please see the {read_}copy_update() in the RCU patches that I linked.
> > In short, you don't wait a grace for mutable access, since in RCU,
> > readers don't block updaters, but instead updater will copy the object,
> > atomically update the pointer and then get an `RcuOld`,
> > which you can either synchronize_rcu() or {call,kfree}_rcu().
> 
> Hm, ok. I don't really need that. What I want rcu for is the internal
> maple tree data structure, so mtree_load() doesn't need to block on the
> maple tree internal spinlock. The contents of the box would be protected
> by a separate lock (probably via LockedBy).
> 

You mean after `load_rcu()`, we could access mutably by a lock? You need
to hold that lock and the rcu_read_lock() while mutably accessing the
return of `load_rcu()`, right? That is basically using RCU as a proof
for existence.

Regards,
Boqun