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

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Date: Fri, 22 Mar 2024 19:26:28 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	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 Fri, Mar 22, 2024 at 10:07:31PM -0400, Kent Overstreet wrote:
[...]
> > Boqun already mentioned the "mixing access sizes", which is actually
> > quite fundamental in the kernel, where we play lots of games with that
> > (typically around locking, where you find patterns line unlock writing
> > a zero to a single byte, even though the whole lock data structure is
> > a word). And sometimes the access size games are very explicit (eg
> > lib/lockref.c).
> 
> I don't think mixing access sizes should be a real barrier. On the read

Well, it actually is, since mixing access sizes is, guess what,
an undefined behavior:

(example in https://doc.rust-lang.org/std/sync/atomic/#memory-model-for-atomic-accesses)

	thread::scope(|s| {
	    // This is UB: using different-sized atomic accesses to the same data
	    s.spawn(|| atomic.store(1, Ordering::Relaxed));
	    s.spawn(|| unsafe {
		let differently_sized = transmute::<&AtomicU16, &AtomicU8>(&atomic);
		differently_sized.store(2, Ordering::Relaxed);
	    });
	});

Of course, you can say "I will just ignore the UB", but if you have to
ignore "compiler rules" to make your code work, why bother use compiler
builtin in the first place? Being UB means they are NOT guaranteed to
work.

> side we can obviously do that with a helper; the write side needs
> compiler help, but "writing just a byte out of a word" is no different
> from a compiler POV that "write a single bit", and we can already mix
> atomic_or() with atomic_add(), with both C atomics and LKMM atomics.
> 

I totally agree with your reasoning here, but maybe the standard doesn't
;-)

Regards,
Boqun