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

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Message-ID: <ZmtC7h7v1t6XJ6EI@boqun-archlinux>
Date: Thu, 13 Jun 2024 12:05:18 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Miguel Ojeda <miguel.ojeda.sandonis@...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>,
	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 Thu, Jun 13, 2024 at 07:22:54PM +0200, Miguel Ojeda wrote:
> On Thu, Jun 13, 2024 at 6:31 PM Boqun Feng <boqun.feng@...il.com> wrote:
> >
> > So let's start with some basic and simple until we really have a need
> > for generic `Atomic<T>`. Thoughts?
> 
> I don't want to delay this, but using generics would be more flexible,
> right? e.g. it could allow us to have atomics of, say, newtypes, if
> that were to be useful.
> 
> Is there a particular disadvantage of using the generics? The two
> cases you mentioned would just be written explicitly, right?
> 
> One disadvantage would be that they are different from the Rust
> standard library ones, e.g. in case we wanted third-party code to use
> them, but could be provided if needed later on.
> 

Well, the other thing is AtomicI32 -> atomic_t and AtomicI64 ->
atomic64_t are perfect mappings, and we can treat AtomicI32 and
AtomicI64 as a separate layer that wires C atomics into Rust. As I said,
we can build `Atomic<T>` on top of this layer, like:

    Atomic<T>
      |
      V
    AtomicI{32,64}
      |
      V
    atomic{,64}_t

and if we drop this layer, the dependencies become:

    Atomic<i32,i64> <- Atomic<u32,u64>
      |
      V
    atomic{,64}_t

i.e. in the same layer of Atomic<T>, some of them directly depend on
some other Atomic<T> types, which doesn't look very clean to me. And it
might be difficult for architecture maintainers to track the exact
dependency for Rust code.

This is also the reason why I didn't use Rust macros to generate
AtomicI32 and AtomicI64 implementation: I use a script to generate .rs
file. This ensures AtomicI32 and AtomicI64 stay with the exact same set
of APIs as atomic{,64}_t (described by scripts/atomic/atomics.tbl. Put
it in another way, I guess you can think AtomicI32 and AtomicI64 as some
sort of intrinsic layer provided by C. And should we need it, we can
build an Atomic<T> layer on top of it.

Does this make sense?

Regards,
Boqun

> Cheers,
> Miguel