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

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Message-ID: <iurfeuqq5hfwhv66d2wozlzv24avyypgtgoqpmorghmimzqwur@zj2qfot47d76>
Date: Wed, 27 Mar 2024 17:49:41 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Boqun Feng <boqun.feng@...il.com>
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 02:21:03PM -0700, Boqun Feng wrote:
> On Wed, Mar 27, 2024 at 03:41:16PM -0400, Kent Overstreet wrote:
> > On Wed, Mar 27, 2024 at 12:07:26PM -0700, Linus Torvalds wrote:
> > > On Wed, 27 Mar 2024 at 11:51, Kent Overstreet <kent.overstreet@...ux.dev> wrote:
> > > >
> > > > On Wed, Mar 27, 2024 at 09:16:09AM -0700, comex wrote:
> > > > > Meanwhile, Rust intentionally lacks strict aliasing.
> > > >
> > > > I wasn't aware of this. Given that unrestricted pointers are a real
> > > > impediment to compiler optimization, I thought that with Rust we were
> > > > finally starting to nail down a concrete enough memory model to tackle
> > > > this safely. But I guess not?
> > > 
> > > Strict aliasing is a *horrible* mistake.
> > > 
> > > It's not even *remotely* "tackle this safely". It's the exact
> > > opposite. It's completely broken.
> > > 
> > > Anybody who thinks strict aliasing is a good idea either
> > > 
> > >  (a) doesn't understand what it means
> > > 
> > >  (b) has been brainwashed by incompetent compiler people.
> > > 
> > > it's a horrendous crock that was introduced by people who thought it
> > > was too complicated to write out "restrict" keywords, and that thought
> > > that "let's break old working programs and make it harder to write new
> > > programs" was a good idea.
> > 
> > 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
unsafe Rust very much has to follow the aliasing rules too.