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

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Date: Fri, 14 Jun 2024 18:33:10 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: 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 Fri, Jun 14, 2024 at 09:22:24PM +0000, Benno Lossin wrote:
> On 14.06.24 16:33, Boqun Feng wrote:
> > On Fri, Jun 14, 2024 at 11:59:58AM +0200, Miguel Ojeda wrote:
> >> On Thu, Jun 13, 2024 at 9:05 PM Boqun Feng <boqun.feng@...il.com> wrote:
> >>>
> >>> Does this make sense?
> >>
> >> Implementation-wise, if you think it is simpler or more clear/elegant
> >> to have the extra lower level layer, then that sounds fine.
> >>
> >> However, I was mainly talking about what we would eventually expose to
> >> users, i.e. do we want to provide `Atomic<T>` to begin with? If yes,
> > 
> > The truth is I don't know ;-) I don't have much data on which one is
> > better. Personally, I think AtomicI32 and AtomicI64 make the users have
> > to think about size, alignment, etc, and I think that's important for
> > atomic users and people who review their code, because before one uses
> > atomics, one should ask themselves: why don't I use a lock? Atomics
> > provide the ablities to do low level stuffs and when doing low level
> > stuffs, you want to be more explicit than ergonomic.
> 
> How would this be different with `Atomic<i32>` and `Atomic<i64>`? Just

The difference is that with Atomic{I32,I64} APIs, one has to choose (and
think about) the size when using atomics, and cannot leave that option
open. It's somewhere unconvenient, but as I said, atomics variables are
different. For example, if someone is going to implement a reference
counter struct, they can define as follow:

	struct Refcount<T> {
	    refcount: AtomicI32,
	    data: UnsafeCell<T>
	}

but with atomic generic, people can leave that option open and do:

	struct Refcount<R, T> {
	    refcount: Atomic<R>,
	    data: UnsafeCell<T>
	}

while it provides configurable options for experienced users, but it
also provides opportunities for sub-optimal types, e.g. Refcount<u8, T>:
on ll/sc architectures, because `data` and `refcount` can be in the same
machine-word, the accesses of `refcount` are affected by the accesses of
`data`.

The point I'm trying to make here is: when you are using atomics, you
care about performance a lot (otherwise, why don't you use a lock?), and
because of that, you should care about the size of the atomics, because
it may affect the performance significantly.

Regards,
Boqun

> because the underlying `Atomic<I>` type is generic shouldn't change
> this, right?
> 
> ---
> Cheers,
> Benno
>