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

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Message-ID: <bjorlxatlpzjlh6dfulham3u4mqsfqt7ir5wtayacaoefr2r7x@lmfcqzcobl3f>
Date: Wed, 27 Mar 2024 15:41:16 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: comex <comexk@...il.com>, "Dr. David Alan Gilbert" <dave@...blig.org>, 
	Philipp Stanner <pstanner@...hat.com>, Boqun Feng <boqun.feng@...il.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 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
problem. A lot of C programmers have stockholm syndrome when it comes to
this, we end up writing a lot of code in weirdly baroque and artificial
styles to partially work around this when we care about performance -
saving things into locals because at least the _stack_ generally can't
alias to avoid forced reloads, or passing and returning things by
reference instead of by value when that's _not the semantics we want_
because otherwise the compiler is going to do an unnecessary copy -
again, that's fundamentally because of aliasing.