From: Philipp Stanner <pstanner@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
Kent Overstreet <kent.overstreet@linux.dev>
Cc: "Boqun Feng" <boqun.feng@gmail.com>,
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>,
"Nicholas Piggin" <npiggin@gmail.com>,
"David Howells" <dhowells@redhat.com>,
"Jade Alglave" <j.alglave@ucl.ac.uk>,
"Luc Maranget" <luc.maranget@inria.fr>,
"Paul E. McKenney" <paulmck@kernel.org>,
"Akira Yokosawa" <akiyks@gmail.com>,
"Daniel Lustig" <dlustig@nvidia.com>,
"Joel Fernandes" <joel@joelfernandes.org>,
"Nathan Chancellor" <nathan@kernel.org>,
"Nick Desaulniers" <ndesaulniers@google.com>,
kent.overstreet@gmail.com,
"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
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: Mon, 25 Mar 2024 14:56:58 +0100 [thread overview]
Message-ID: <c51227c9a4103ad1de43fc3cda5396b1196c31d7.camel@redhat.com> (raw)
In-Reply-To: <CAHk-=whkQk=zq5XiMcaU3xj4v69+jyoP-y6Sywhq-TvxSSvfEA@mail.gmail.com>
On Fri, 2024-03-22 at 17:36 -0700, Linus Torvalds wrote:
> On Fri, 22 Mar 2024 at 17:21, Kent Overstreet
> <kent.overstreet@linux.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
>
next prev parent reply other threads:[~2024-03-25 13:57 UTC|newest]
Thread overview: 76+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-22 23:38 [WIP 0/3] Memory model and atomic API in Rust Boqun Feng
2024-03-22 23:38 ` [WIP 1/3] rust: Introduce atomic module Boqun Feng
2024-03-22 23:52 ` Andrew Lunn
2024-03-23 0:03 ` Boqun Feng
2024-03-23 19:13 ` Miguel Ojeda
2024-03-23 19:30 ` Boqun Feng
2024-03-23 9:58 ` Alice Ryhl
2024-03-23 14:10 ` Andrew Lunn
2024-03-23 19:09 ` Miguel Ojeda
2024-03-26 5:56 ` Trevor Gross
2024-03-22 23:38 ` [WIP 2/3] rust: atomic: Add ARM64 fetch_add_relaxed() Boqun Feng
2024-03-22 23:38 ` [WIP 3/3] rust: atomic: Add fetch_sub_release() Boqun Feng
2024-03-22 23:57 ` [WIP 0/3] Memory model and atomic API in Rust Kent Overstreet
2024-03-23 0:12 ` Linus Torvalds
2024-03-23 0:21 ` Kent Overstreet
2024-03-23 0:36 ` Linus Torvalds
2024-03-23 2:07 ` Kent Overstreet
2024-03-23 2:26 ` Boqun Feng
2024-03-23 2:33 ` Kent Overstreet
2024-03-23 2:57 ` Boqun Feng
2024-03-23 3:10 ` Kent Overstreet
2024-03-23 3:51 ` Boqun Feng
2024-03-23 4:16 ` Kent Overstreet
2024-03-25 13:56 ` Philipp Stanner [this message]
2024-03-25 17:44 ` Linus Torvalds
2024-03-25 18:59 ` Kent Overstreet
2024-03-25 19:44 ` Linus Torvalds
2024-03-25 21:14 ` Kent Overstreet
2024-03-25 21:37 ` Boqun Feng
2024-03-25 22:09 ` Kent Overstreet
2024-03-25 22:38 ` Boqun Feng
2024-03-25 23:02 ` Kent Overstreet
2024-03-25 23:41 ` Boqun Feng
2024-03-26 0:05 ` Dr. David Alan Gilbert
2024-03-26 0:36 ` Kent Overstreet
2024-03-26 1:35 ` Dr. David Alan Gilbert
2024-03-26 3:28 ` Kent Overstreet
2024-03-26 2:51 ` Boqun Feng
2024-03-26 3:49 ` Linus Torvalds
2024-03-26 14:35 ` Dr. David Alan Gilbert
2024-03-27 16:16 ` comex
2024-03-27 18:50 ` Kent Overstreet
2024-03-27 19:07 ` Linus Torvalds
2024-03-27 19:41 ` Kent Overstreet
2024-03-27 20:45 ` Linus Torvalds
2024-03-27 21:41 ` Kent Overstreet
2024-03-27 22:57 ` Linus Torvalds
2024-03-27 23:35 ` Kent Overstreet
2024-03-27 21:21 ` Boqun Feng
2024-03-27 21:49 ` Kent Overstreet
2024-03-27 22:26 ` Boqun Feng
2024-03-27 21:56 ` comex
2024-03-27 22:02 ` comex
2024-04-05 17:13 ` Philipp Stanner
2024-04-08 16:02 ` Matthew Wilcox
2024-04-08 16:55 ` Paul E. McKenney
2024-04-08 17:03 ` Matthew Wilcox
2024-04-08 18:47 ` Paul E. McKenney
2024-04-09 0:58 ` Kent Overstreet
2024-04-09 4:47 ` Paul E. McKenney
2024-04-08 17:01 ` Linus Torvalds
2024-04-08 18:14 ` Al Viro
2024-04-08 20:05 ` Linus Torvalds
2024-03-23 21:40 ` comex
2024-03-24 15:22 ` Alan Stern
2024-03-24 17:37 ` comex
2024-03-23 0:15 ` Boqun Feng
2024-03-23 0:49 ` Boqun Feng
2024-03-23 1:42 ` Kent Overstreet
2024-03-23 14:29 ` Andrew Lunn
2024-03-23 14:41 ` Boqun Feng
2024-03-23 14:55 ` Boqun Feng
2024-03-25 10:44 ` Mark Rutland
2024-03-25 20:59 ` Boqun Feng
2024-04-09 10:50 ` Peter Zijlstra
2024-04-16 18:12 ` Boqun Feng
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