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

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Date: Wed, 27 Mar 2024 15:26:07 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,	comex <comexk@...il.com>,
	"Dr. David Alan Gilbert" <dave@...blig.org>,
	Philipp Stanner <pstanner@...hat.com>,
	rust-for-linux <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>,
	Marco Elver <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 Wed, Mar 27, 2024 at 05:49:41PM -0400, Kent Overstreet wrote:
[...]
> > > Strict aliasing is crap in C and C++ because we started out with
> > > unrestricetd pointers, and it just doesn't work in C and C++ with the
> > > realities of the kind of code we have to write, and we never got any
> > > kind of a model that would have made it workable. Never mind trying to
> > > graft that onto existing codebases...
> > > 
> > > (Restrict was crap too... no scoping, nothing but a single f*cking
> > > keyword? Who ever thought _that_ was going to work?)
> > > 
> > > _But_: the lack of any aliasing guarantees means that writing through
> > > any pointer can invalidate practically anything, and this is a real
> > 
> > I don't know whether I'm 100% correct on this, but Rust has references,
> > so things like "you have a unique reference to a part of memory, no one
> > would touch it in the meanwhile" are represented by `&mut`, to get a
> > `&mut` from a raw pointer, you need unsafe, where programmers can
> > provide the reasoning of the safety of the accesses. More like "pointers
> > can alias anyone but references cannot" to me.
> 
> That's not really a workable rule because in practice every data
> structure has unsafe Rust underneath. Strict aliasing would mean that

I don't follow, a plain data structure like:

	struct Point { x: i64, y: i64 }

doesn't have any unsafe Rust underneath I think.

> unsafe Rust very much has to follow the aliasing rules too.
> 

The point I was trying to say, the aliasing rule on Rust raw pointers is 
relatively relaxed compared to strict aliasing in C since Rust has
references which should have more accurate informatio on aliasing.

(but not a language expert).

Regards,
Boqun