linux-kernel - Re: [PATCH 4/5] rust: sync: atomic: Add Atomic<*mut T> support

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Message-ID: <aW9tmbVWaqSVOiD8@tardis-2.local>
Date: Tue, 20 Jan 2026 19:57:13 +0800
From: Boqun Feng <boqun.feng@...il.com>
To: Gary Guo <gary@...yguo.net>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
	rcu@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>, Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Mark Rutland <mark.rutland@....com>,
	"Paul E. McKenney" <paulmck@...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>,
	FUJITA Tomonori <fujita.tomonori@...il.com>
Subject: Re: [PATCH 4/5] rust: sync: atomic: Add Atomic<*mut T> support

On Sun, Jan 18, 2026 at 03:39:40PM +0000, Gary Guo wrote:
[...]
> >> 
> >> How about *const T?
> >> 
> >
> > In general I want to avoid const raw pointers since it provides very
> > little extra compared to mut raw pointers. For compiler optimization,
> > provenenace is more important than "const vs mut" modifier, for
> > dereference, it's unsafe anyway and users need to provide reasoning
> > (including knowing the provenance and other accesses may happen to the
> > same address), so I feel the type difference of "*const T" vs "*mut T"
> > doesn't do anything extra either.
> >
> > Think about it, in Rust std, there are two pointer types only maps to
> > "*mut T": NonNull<T> (as_ptr() returns a `*mut T`) and AtomicPtr<T>
> > (as_ptr() returns a `*mut *mut T`). And there is no type like
> > NonNullConst<T> and AtomicConstPtr<T>. This is a lint to me that we may
> > not need to support `*const T` in most cases.
> 
> Actually `NonNull` is internally `*const T`, because it's covariant, unlike
> `*mut T` which is invariant.
> 

Ah, right!

> Now, for atomics, it's less likely that you actually want covariance. So this
> difference matters less.
> 

Agreed.

> >
> > But maybe I'm missing something? If you have a good reason, we can
> > obviously add the support for `*const T`.
> 
> It just feels that it is somewhat inconsistent. There's no good motivation right
> now. I am fine to leave it out and add when needed.
> 

Yeah, we can also add it later. Thanks!

Regards,
Boqun

> Best,
> Gary