linux-kernel - Re: [RFC 2/2] rust: sync: Add atomic support

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Message-ID: <20240614095124.GN8774@noisy.programming.kicks-ass.net>
Date: Fri, 14 Jun 2024 11:51:24 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Gary Guo <gary@...yguo.net>, 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>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
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	Alice Ryhl <aliceryhl@...gle.com>,
	Alan Stern <stern@...land.harvard.edu>,
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	Jade Alglave <j.alglave@....ac.uk>,
	Luc Maranget <luc.maranget@...ia.fr>,
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	linux-fsdevel@...r.kernel.org, Trevor Gross <tmgross@...ch.edu>,
	dakr@...hat.com
Subject: Re: [RFC 2/2] rust: sync: Add atomic support

On Thu, Jun 13, 2024 at 09:30:26AM -0700, Boqun Feng wrote:

> We can always add a layer on top of what we have here to provide the
> generic `Atomic<T>`. However, I personally don't think generic
> `Atomic<T>` is a good idea, for a few reasons:
> 
> *	I'm not sure it will bring benefits to users, the current atomic
> 	users in kernel are pretty specific on the size of atomic they
> 	use, so they want to directly use AtomicI32 or AtomicI64 in
> 	their type definitions rather than use a `Atomic<T>` where their
> 	users can provide type later.
> 
> *	I can also see the future where we have different APIs on
> 	different types of atomics, for example, we could have a:
> 
> 		impl AtomicI64 {
> 		    pub fn split(&self) -> (&AtomicI32, &AtomicI32)
> 		}
> 
> 	which doesn't exist for AtomicI32. Note this is not a UB because
> 	we write our atomic implementation in asm, so it's perfectly
> 	fine for mix-sized atomics.
> 
> So let's start with some basic and simple until we really have a need
> for generic `Atomic<T>`. Thoughts?

Not on the generic thing, but on the lack of long. atomic_long_t is
often used when we have pointers with extra bits on. Then you want a
number type in order to be able to manipulate the low bits.