linux-kernel - Re: [RFC] rust: types: Add read_once and write

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Date:   Wed, 25 Oct 2023 20:31:37 -0700
From:   Boqun Feng <boqun.feng@...il.com>
To:     Benno Lossin <benno.lossin@...ton.me>
Cc:     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>,
        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>,
        Tom Rix <trix@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        kent.overstreet@...il.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        elver@...gle.com, Matthew Wilcox <willy@...radead.org>,
        Dave Chinner <david@...morbit.com>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [RFC] rust: types: Add read_once and write_once

On Wed, Oct 25, 2023 at 04:02:45PM -0700, Boqun Feng wrote:
[...]
> > > +/// The counter part of C `READ_ONCE()`.
> > > +///
> > > +/// The semantics is exactly the same as `READ_ONCE()`, especially when used for intentional data
> > > +/// races.
> > 
> > It would be great if these semantics are either linked or spelled out
> > here. Ideally both.
> > 
> 
> Actually I haven't found any document about `READ_ONCE()` races with
> writes is not UB: we do have document saying `READ_ONCE()` will disable
> KCSAN checks, but no document says (explicitly) that it's not a UB.
> 

Apparently I wasn't carefully reading the doc, in
tools/memory-model/Documentation/explanation.txt, there is:

	In technical terms, the compiler is allowed to assume that when the
	program executes, there will not be any data races.  A "data race"
	occurs when there are two memory accesses such that:

	1.      they access the same location,

	2.      at least one of them is a store,

	3.      at least one of them is plain,

	4.      they occur on different CPUs (or in different threads on the
		same CPU), and

	5.      they execute concurrently.

the #3 limits that in LKMM, data races cannot happen if both accesses
are marked (i.e. not plain).

Thank Paul for bringing this to me, and I will update this accordingly
in the next version.

Regards,
Boqun

> > > +///
> > > +/// # Safety
> > > +///
> > > +/// * `src` must be valid for reads.
> > > +/// * `src` must be properly aligned.
> > > +/// * `src` must point to a properly initialized value of value `T`.
> > > +#[inline(always)]
> > > +pub unsafe fn read_once<T: Copy>(src: *const T) -> T {
> > 
> > Why only `T: Copy`?
> > 
[...]