linux-kernel - Re: [PATCH v3 06/10] rust: list: add List

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Message-ID: <CAH5fLgjNCXwhUaHh7dxm-5LmFe-TtiizK2cWscSp9bkdknRywQ@mail.gmail.com>
Date: Thu, 1 Aug 2024 11:40:57 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: Miguel Ojeda <ojeda@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Andreas Hindborg <a.hindborg@...sung.com>, Marco Elver <elver@...gle.com>, Coly Li <colyli@...e.de>, 
	Paolo Abeni <pabeni@...hat.com>, Pierre Gondois <pierre.gondois@....com>, 
	Ingo Molnar <mingo@...nel.org>, Jakub Kicinski <kuba@...nel.org>, Wei Yang <richard.weiyang@...il.com>, 
	Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v3 06/10] rust: list: add List

On Thu, Aug 1, 2024 at 11:11 AM Benno Lossin <benno.lossin@...ton.me> wrote:
>
> On 23.07.24 10:22, Alice Ryhl wrote:
> > +    /// Add the provided item to the back of the list.
> > +    pub fn push_back(&mut self, item: ListArc<T, ID>) {
> > +        let raw_item = ListArc::into_raw(item);
> > +        // SAFETY:
> > +        // * We just got `raw_item` from a `ListArc`, so it's in an `Arc`.
> > +        // * If this requirement is violated, then the previous caller of `prepare_to_insert`
> > +        //   violated the safety requirement that they can't give up ownership of the `ListArc`
> > +        //   until they call `post_remove`.
>
> I don't like this negative phrasing, what about "Since we have ownership
> of the `ListArc`, `post_remove` must have been called after each
> previous call to `prepare_to_insert`."?

I think we just need to argue about the most recent call to
prepare_to_insert but ok.

> > +        // * We own the `ListArc`.
> > +        // * Removing items from this list is always done using `remove_internal_inner`, which
> > +        //   calls `post_remove` before giving up ownership.
> > +        let list_links = unsafe { T::prepare_to_insert(raw_item) };
> > +        // SAFETY: We have not yet called `post_remove`, so `list_links` is still valid.
> > +        let item = unsafe { ListLinks::fields(list_links) };
> > +
> > +        if self.first.is_null() {
> > +            self.first = item;
> > +            // SAFETY: The caller just gave us ownership of these fields.
> > +            // INVARIANT: A linked list with one item should be cyclic.
> > +            unsafe {
> > +                (*item).next = item;
> > +                (*item).prev = item;
> > +            }
> > +        } else {
> > +            let next = self.first;
> > +            // SAFETY: By the type invariant, this pointer is valid or null. We just checked that
> > +            // it's not null, so it must be valid.
> > +            let prev = unsafe { (*next).prev };
> > +            // SAFETY: Pointers in a linked list are never dangling, and the caller just gave us
> > +            // ownership of the fields on `item`.
> > +            // INVARIANT: This correctly inserts `item` between `prev` and `next`.
> > +            unsafe {
> > +                (*item).next = next;
> > +                (*item).prev = prev;
> > +                (*prev).next = item;
> > +                (*next).prev = item;
> > +            }
>
> You have this pattern several times, maybe make a function for this?

It's just two times. I think it's fine.

> > +        if !next.is_null() {
> > +            // This is really a no-op, but this ensures that `item` is a raw pointer that was
> > +            // obtained without going through a pointer->reference->pointer conversion rountrip.
> > +            // This ensures that the list is valid under the more restrictive strict provenance
> > +            // ruleset.
> > +            //
> > +            // SAFETY: We just checked that `next` is not null, and it's not dangling by the
> > +            // list invariants.
> > +            unsafe {
> > +                debug_assert_eq!(item, (*next).prev);
> > +                item = (*next).prev;
> > +            }
>
> How bad do you reckon is this for performance?

I don't think it's a problem at all.

Alice