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

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Date: Mon, 25 Mar 2024 14:59:34 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Philipp Stanner <pstanner@...hat.com>, 
	Boqun Feng <boqun.feng@...il.com>, 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 Mon, Mar 25, 2024 at 10:44:43AM -0700, Linus Torvalds wrote:
> On Mon, 25 Mar 2024 at 06:57, Philipp Stanner <pstanner@...hat.com> wrote:
> >
> > On Fri, 2024-03-22 at 17:36 -0700, Linus Torvalds wrote:
> > >
> > > It's kind of like our "volatile" usage. If you read the C (and C++)
> > > standards, you'll find that you should use "volatile" on data types.
> > > That's almost *never* what the kernel does. The kernel uses
> > > "volatile"
> > > in _code_ (ie READ_ONCE() etc), and uses it by casting etc.
> > >
> > > Compiler people don't tend to really like those kinds of things.
> >
> > Just for my understanding: Why don't they like it?
> 
> So I actually think most compiler people are perfectly fine with the
> kernel model of mostly doing 'volatile' not on the data structures
> themselves, but as accesses through casts.
> 
> It's very traditional C, and there's actually nothing particularly odd
> about it. Not even from a compiler standpoint.
> 
> In fact, I personally will argue that it is fundamentally wrong to
> think that the underlying data has to be volatile. A variable may be
> entirely stable in some cases (ie locks held), but not in others.
> 
> So it's not the *variable* (aka "object") that is 'volatile', it's the
> *context* that makes a particular access volatile.
> 
> That explains why the kernel has basically zero actual volatile
> objects, and 99% of all volatile accesses are done through accessor
> functions that use a cast to mark a particular access volatile.
> 
> But I've had negative comments from compiler people who read the
> standards as language lawyers (which honestly, I despise - it's always
> possible to try to argue what the meaning of some wording is), and
> particularly C++ people used to be very very antsy about "volatile".
> 
> They had some truly _serious_ problems with volatile.
> 
> The C++ people spent absolutely insane amounts of time arguing about
> "volatile objects" vs "accesses", and how an access through a cast
> didn't make the underlying object volatile etc.

To be fair, "volatile" dates from an era when we didn't have the haziest
understanding of what a working memory model for C would look like or
why we'd even want one.

(Someone might want to think about depracating volatile on objects and
adding compiler flag to disable it; I suspect it'd be a useful cleanup
for the compiler guys if they could get rid of it.)

The way the kernel uses volatile in e.g. READ_ONCE() is fully in line
with modern thinking, just done with the tools available at the time. A
more modern version would be just

__atomic_load_n(ptr, __ATOMIC_RELAXED)

Except C atomics only support pointer and integer types, READ_ONCE()
supports anything up to machine word size - that's something the C
people need to fix.