Feedback-ID: iad51458e:Fastmail
Date:   Fri, 27 Jan 2023 14:09:41 -0800
From:   Boqun Feng <boqun.feng@gmail.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Jules Maselbas <jmaselbas@kalray.eu>,
        Will Deacon <will@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Arnd Bergmann <arnd@arndb.de>, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Alan Stern <stern@rowland.harvard.edu>,
        Andrea Parri <parri.andrea@gmail.com>,
        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>,
        Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>,
        Hernan Ponce de Leon <hernan.poncedeleon@huaweicloud.com>,
        Paul =?iso-8859-1?Q?Heidekr=FCger?= <paul.heidekrueger@in.tum.de>,
        Marco Elver <elver@google.com>,
        Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Gary Guo <gary@garyguo.net>,
        =?iso-8859-1?Q?Bj=F6rn?= Roy Baron <bjorn3_gh@protonmail.com>
Subject: Re: [PATCH] locking/atomic: atomic: Use arch_atomic_{read,set} in
 generic atomic ops
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On Fri, Jan 27, 2023 at 03:34:33PM +0100, Peter Zijlstra wrote:
> > I also noticed that GCC has some builtin/extension to do such things,
> > __atomic_OP_fetch and __atomic_fetch_OP, but I do not know if this
> > can be used in the kernel.
> 
> On a per-architecture basis only, the C/C++ memory model does not match
> the Linux Kernel memory model so using the compiler to generate the
> atomic ops is somewhat tricky and needs architecture audits.

Hijack this thread a little bit, but while we are at it, do you think it
makes sense that we have a config option that allows archs to
implement LKMM atomics via C11 (volatile) atomics? I know there are gaps
between two memory models, but the option is only for fallback/generic
implementation so we can put extra barriers/orderings to make things
guaranteed to work.

It'll be a code version of this document:

	https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p0124r7.html

(although I realise there may be a few mistakes in that doc since I
wasn't familiar with C11 memory model when I wrote part of the doc, but
these can be fixed)

Another reason I ask is that since Rust is coming, we need to provide
our LKMM atomics in Rust so that C code and Rust code can talk via same
atomic variables, since both sides need to use the same memory model.
My choices are:

1.	Using FFI to call Linux atomic APIs: not inline therefore not
	efficient.

2.	Implementing Rust LKMM atomics in asm: much more work although
	I'm OK if we have to do it.

3.	Implementing Rust LKMM atomics with standard atomics (i.e. C/C++
	atomics):

	*	Requires Rust has "volatile" atomics, which is WIP but
		looks promising
	
	*	Less efficient compared to choice #2 but more efficient
		compared to choice #1

Ideally, choice #2 is the best option for all architectures, however, if
we have the generic implementation based on choice #3, for some archs it
may be good enough.

Thoughts?

[Cc LKMM and Rust people]

Regards,
Boqun