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

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Date: Sun, 16 Jun 2024 07:35:57 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Benno Lossin <benno.lossin@...ton.me>,
	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
	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 07:16:30AM -0700, Boqun Feng wrote:
> On Sun, Jun 16, 2024 at 05:51:07AM -0400, Kent Overstreet wrote:
> > On Sat, Jun 15, 2024 at 03:12:33PM -0700, Boqun Feng wrote:
> > > What's the issue of having AtomicI32 and AtomicI64 first then? We don't
> > > need to do 1 or 2 until the real users show up.
> > > 
> > > And I'd like also to point out that there are a few more trait bound
> > > designs needed for Atomic<T>, for example, Atomic<u32> and Atomic<i32>
> > > have different sets of API (no inc_unless_negative() for u32).
> > > 
> > > Don't make me wrong, I have no doubt we can handle this in the type
> > > system, but given the design work need, won't it make sense that we take
> > > baby steps on this? We can first introduce AtomicI32 and AtomicI64 which
> > > already have real users, and then if there are some values of generic
> > > atomics, we introduce them and have proper discussion on design.
> > > 
> > > To me, it's perfectly fine that Atomic{I32,I64} co-exist with Atomic<T>.
> > > What's the downside? A bit specific example would help me understand
> > > the real concern here.
> > 
> > Err, what?
> > 
> > Of course we want generic atomics, and we need that for properly
> > supporting cmpxchg.
> > 
> 
> Nope. Note this series only introduces the atomic types (atomic_ C
> APIs), but cmpxchg C APIs (no atomic_ prefix) are probably presented via
> a different API, where we need to make it easier to interact with normal
> types, and we may use generic there.
> 

Or it could be a generic function instead of generic type like:

	pub unsafe fn cmpxchg<T>(ptr: * mut T, old: T, new T) -> T

the "unsafe" part is due to `ptr` may not be a valid pointer or this may
make normal accesses data race.

Regards,
Boqun

> > Bogun, you've got all the rust guys pushing for doing this with
> > generics, I'm not sure why you're being stubborn here?
> 
> Hmm? Have you seen the email I replied to John, a broader Rust community
> seems doesn't appreciate the idea of generic atomics.
> 
> Regards,
> Boqun