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

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Message-ID: <c51227c9a4103ad1de43fc3cda5396b1196c31d7.camel@redhat.com>
Date: Mon, 25 Mar 2024 14:56:58 +0100
From: Philipp Stanner <pstanner@...hat.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>, Kent Overstreet
	 <kent.overstreet@...ux.dev>
Cc: 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 Fri, 2024-03-22 at 17:36 -0700, Linus Torvalds wrote:
> On Fri, 22 Mar 2024 at 17:21, Kent Overstreet
> <kent.overstreet@...ux.dev> wrote:
> > 
> > Besides that there's cross arch support to think about - it's hard
> > to
> > imagine us ever ditching our own atomics.
> 
> > [... SNIP ...]
> > 
> > I was thinking about something more incremental - just an optional
> > mode
> > where our atomics were C atomics underneath. It'd probably give the
> > compiler people a much more effective way to test their stuff than
> > anything they have now.
> 
> I suspect it might be painful, and some compiler people would throw
> their hands up in horror, because the C++ atomics model is based
> fairly solidly on atomic types, and the kernel memory model is much
> more fluid.
> 
> Boqun already mentioned the "mixing access sizes", which is actually
> quite fundamental in the kernel, where we play lots of games with
> that
> (typically around locking, where you find patterns line unlock
> writing
> a zero to a single byte, even though the whole lock data structure is
> a word). And sometimes the access size games are very explicit (eg
> lib/lockref.c).
> 
> But it actually goes deeper than that. While we do have "atomic_t"
> etc
> for arithmetic atomics, and that probably would map fairly well to
> C++
> atomics, in other cases we simply base our atomics not on _types_,
> but
> on code.
> 
> IOW, we do things like "cmpxchg()", and the target of that atomic
> access is just a regular data structure field.
> 
> 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?

I guess since compiler people have to support volatile pointers
anyways, temporarily casting something to such a volatile pointer
shouldn't be a problem either – so they don't dislike it because it's
more difficult to implement, but because it's more difficult to verify
for correctness?


P.

> 
>             Linus
>