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From: Alice Ryhl <aliceryhl@google.com>
Date: Thu, 4 Apr 2024 16:03:33 +0200
Message-ID: <CAH5fLghJtSvW-zNnDNFVgtb-px2d4W-xUk2++QdvyMYFMBGM-Q@mail.gmail.com>
Subject: Re: [PATCH 3/9] rust: list: add struct with prev/next pointers
To: Benno Lossin <benno.lossin@proton.me>
Cc: Miguel Ojeda <ojeda@kernel.org>, Andrew Morton <akpm@linux-foundation.org>, 
	Alex Gaynor <alex.gaynor@gmail.com>, Wedson Almeida Filho <wedsonaf@gmail.com>, 
	Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>, 
	=?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, 
	Andreas Hindborg <a.hindborg@samsung.com>, Marco Elver <elver@google.com>, 
	Kees Cook <keescook@chromium.org>, Coly Li <colyli@suse.de>, Paolo Abeni <pabeni@redhat.com>, 
	Pierre Gondois <pierre.gondois@arm.com>, Ingo Molnar <mingo@kernel.org>, 
	Jakub Kicinski <kuba@kernel.org>, Wei Yang <richard.weiyang@gmail.com>, 
	Matthew Wilcox <willy@infradead.org>, linux-kernel@vger.kernel.org, 
	rust-for-linux@vger.kernel.org
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On Wed, Apr 3, 2024 at 5:57=E2=80=AFPM Benno Lossin <benno.lossin@proton.me=
> wrote:
>
> On 02.04.24 14:17, Alice Ryhl wrote:
> > +/// Implemented by types where a [`ListArc<Self>`] can be inserted int=
o a `List`.
> > +///
> > +/// # Safety
> > +///
> > +/// Implementers must ensure that they provide the guarantees document=
ed on the three methods
> > +/// below.
> > +///
> > +/// [`ListArc<Self>`]: ListArc
> > +pub unsafe trait ListItem<const ID: u64 =3D 0>: ListArcSafe<ID> {
> > +    /// Views the [`ListLinks`] for this value.
> > +    ///
> > +    /// # Guarantees
> > +    ///
> > +    /// * If there is a currently active call to `prepare_to_insert`, =
then this returns the same
> > +    ///   pointer as the one returned by the currently active call to =
`prepare_to_insert`.
>
> I was a bit confused by the term "active call to `prepare_to_insert`",
> since I thought that the function would need to be executed at this
> moment. I inferred from below that you mean by this that there has been
> a `prepare_to_insert` call, but not yet a corresponding `post_remove`
> call.
> I did not yet find a better way to phrase this.
>
> I like putting the guarantees on the functions very much.

How about this?

If there is a previous call to `prepare_to_insert` and there is no
call to `post_remove` since the most recent such call, then this
returns the same pointer as the one returned by the most recent call
to `prepare_to_insert`.

Otherwise, the returned pointer points at a read-only [`ListLinks`]
with two null pointers.

Alice