linux-kernel - Re: [WIP 0/3] Memory model and atomic API in Rust

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Date: Mon, 8 Apr 2024 19:14:36 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Matthew Wilcox <willy@...radead.org>,
	Philipp Stanner <pstanner@...hat.com>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	Boqun Feng <boqun.feng@...il.com>, 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>,
	Gary Guo <gary@...yguo.net>,
	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>,
	linux-arm-kernel@...ts.infradead.org, linux-fsdevel@...r.kernel.org
Subject: Re: [WIP 0/3] Memory model and atomic API in Rust

On Mon, Apr 08, 2024 at 10:01:32AM -0700, Linus Torvalds wrote:

> For our own historical reasons, while we have a few generic atomic
> operations: bit operations, cmpxchg, etc, most of our arithmetic and
> logical ops all rely on a special "atomic_t" type (later extended with
> "atomic_long_t").
> 
> The reason? The garbage that is legacy Sparc atomics.
> 
> Sparc historically basically didn't have any atomics outside of the
> 'test and set byte' one, so if you wanted an atomic counter thing, and
> you cared about sparc, you had to play games with "some bits of the
> counter are the atomic byte lock".
> 
> And we do not care about that Sparc horror any *more*, but we used to.

FWIW, PA-RISC is no better - the same "fetch and replace with constant"
kind of primitive as for sparc32, only the constant is (u32)0 instead
of (u8)~0.  And unlike sparc64, 64bit variant didn't get better.