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

From: Kent Overstreet <kent.overstreet@linux.dev>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: comex <comexk@gmail.com>,
	"Dr. David Alan Gilbert" <dave@treblig.org>,
	"Philipp Stanner" <pstanner@redhat.com>,
	"Boqun Feng" <boqun.feng@gmail.com>,
	rust-for-linux <rust-for-linux@vger.kernel.org>,
	linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
	llvm@lists.linux.dev, "Miguel Ojeda" <ojeda@kernel.org>,
	"Alex Gaynor" <alex.gaynor@gmail.com>,
	"Wedson Almeida Filho" <wedsonaf@gmail.com>,
	"Gary Guo" <gary@garyguo.net>,
	"Björn Roy Baron" <bjorn3_gh@protonmail.com>,
	"Benno Lossin" <benno.lossin@proton.me>,
	"Andreas Hindborg" <a.hindborg@samsung.com>,
	"Alice Ryhl" <aliceryhl@google.com>,
	"Alan Stern" <stern@rowland.harvard.edu>,
	"Andrea Parri" <parri.andrea@gmail.com>,
	"Will Deacon" <will@kernel.org>,
	"Peter Zijlstra" <peterz@infradead.org>,
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	"David Howells" <dhowells@redhat.com>,
	"Jade Alglave" <j.alglave@ucl.ac.uk>,
	"Luc Maranget" <luc.maranget@inria.fr>,
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	kent.overstreet@gmail.com,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Marco Elver" <elver@google.com>,
	"Mark Rutland" <mark.rutland@arm.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [WIP 0/3] Memory model and atomic API in Rust
Date: Wed, 27 Mar 2024 15:41:16 -0400	[thread overview]
Message-ID: <bjorlxatlpzjlh6dfulham3u4mqsfqt7ir5wtayacaoefr2r7x@lmfcqzcobl3f> (raw)
In-Reply-To: <CAHk-=wgQy+FRKjO_BvZgZN56w6-+jDO8p-Mt=X=zM70CG=CVBQ@mail.gmail.com>

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@linux.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.