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

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Date: Sun, 16 Jun 2024 08:54:09 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
	Benno Lossin <benno.lossin@...ton.me>, 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>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Andrea Parri <parri.andrea@...il.com>,	Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nicholas Piggin <npiggin@...il.com>,	David Howells <dhowells@...hat.com>,
	Jade Alglave <j.alglave@....ac.uk>,	Luc Maranget <luc.maranget@...ia.fr>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Akira Yokosawa <akiyks@...il.com>,	Daniel Lustig <dlustig@...dia.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,	kent.overstreet@...il.com,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, elver@...gle.com,
	Mark Rutland <mark.rutland@....com>,
	Thomas Gleixner <tglx@...utronix.de>,	Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Catalin Marinas <catalin.marinas@....com>,	torvalds@...ux-foundation.org,
 linux-arm-kernel@...ts.infradead.org,	linux-fsdevel@...r.kernel.org,
 Trevor Gross <tmgross@...ch.edu>,	dakr@...hat.com
Subject: Re: [RFC 2/2] rust: sync: Add atomic support

On Sun, Jun 16, 2024 at 11:32:31AM -0400, Kent Overstreet wrote:
> On Sun, Jun 16, 2024 at 05:14:56PM +0200, Miguel Ojeda wrote:
> > On Sun, Jun 16, 2024 at 4:16 PM Boqun Feng <boqun.feng@...il.com> wrote:
> > >
> > > Hmm? Have you seen the email I replied to John, a broader Rust community
> > > seems doesn't appreciate the idea of generic atomics.
> > 
> > I don't think we can easily draw that conclusion from those download
> > numbers / dependent crates.
> > 
> > portable-atomic may be more popular simply because it provides
> > features for platforms the standard library does not. The interface
> > being generic or not may have nothing to do with it. Or perhaps
> > because it has a 1.x version, while the other doesn't, etc.
> > 
> > In fact, the atomic crate is essentially about providing `Atomic<T>`,
> > so one could argue that all those downloads are precisely from people
> > that want a generic atomic.
> > 
> > Moreover, I noticed portable-atomic's issue #1 in GitHub is,
> > precisely, adding `Atomic<T>` support. The maintainer has a PR for
> > that updated over time, most recently a few hours ago.
> > 
> > There is also `AtomicCell<T>` from crossbeam, which is the first
> > feature listed in its docs.
> > 
> > Anyway...
> > 
> > The way I see it, both approaches seem similar (i.e. for what we are
> > going to use them for today, at least) and neither apparently has a
> > major downside today for those use cases (apart from needed refactors
> > later to go to another approach).
> > 
> > (By the "generic approach", by the way, I mean just providing
> > `Atomic<{i32,i64}>`, not a complex design)
> > 
> > So it is up to you on what you send for the non-RFC patches, of
> > course, and if nobody has the time / wants to do the work for the
> > "simple" generic approach, then we can just go ahead with this for the
> > moment. But I think it would be nice to at least consider the "simple"
> > generic approach to see how much worse it would be.
> > 
> > Other bits to consider, that perhaps give you arguments for one or the
> > other: consequences on the compilation time, on inlining, on the error
> > messages for new users, on the generated documentation, on how easy to
> > grep they are, etc.
> 
> Yeah, rereading the thread - I'm with Miguel and Gary.
> 
> Generics are simply the correct way to do it, if the wider rust
> community didn't do it that way I think that can be chalked up more to
> historical baggage or needlessly copying the base integer type scheme.
> 
> Let's please do it right here, and generics are the correct approach.

If so, maybe we should do u<Wide> instead of u8, u16, oh, and probably
just Integer<Sign, Wide> instead of i{8,16,32,64) and u{8,16,32,64} ;-)

Regards,
Boqun