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

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Date: Fri, 14 Jun 2024 07:18:30 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
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>,
 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 Fri, Jun 14, 2024 at 11:51:24AM +0200, Peter Zijlstra wrote:
> 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.

I mentioned my plan on AtomicPtr, but I think I should have clarified
this more. My plan is:

pub struct AtomicIsize {
    #[cfg(CONFIG_64BIT)]
    inner: AtomicI64
    #[cfg(not(CONFIG_64BIT))]
    inner: AtomicI32
}

i.e. building AtomicIsize (Rust's atomic_long_t) based on AtomicI64 and
AtomicI32. And we can a AtomicPtr type on it:

pub struct AtomicPtr<T> {
    inner: AtomicIsize,
    _type: PhantomData<*mut T>
}

Of course, I need to do some code generating work for AtomicIsize and
AtomicPtr, I plan to do that in Rust not in scripts, this will keep the
rust/kernel/sync/atomic/impl.rs relatively small (i.e. the Rust/C
interface is smaller). I can include this part in the next version, if
you want to see it.

Regards,
Boqun