Re: [PATCH v3 04/10] rust: list: add struct with prev/next pointers

[Alice Ryhl]

On Thu, Aug 1, 2024 at 12:45 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
>
> On 01.08.24 11:42, Alice Ryhl wrote:
> > On Wed, Jul 31, 2024 at 8:41 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
> >>
> >> On 23.07.24 10:22, Alice Ryhl wrote:
> >>> +/// The prev/next pointers for an item in a linked list.
> >>> +///
> >>> +/// # Invariants
> >>> +///
> >>> +/// The fields are null if and only if this item is not in a list.
> >>> +#[repr(transparent)]
> >>> +pub struct ListLinks<const ID: u64 = 0> {
> >>> +    #[allow(dead_code)]
> >>> +    inner: Opaque<ListLinksFields>,
> >>
> >> Do you really need `Opaque`? Or would `UnsafeCell` be enough? (If it is
> >> enough and you change this, be aware that `Opaque` is `!Unpin`, so if
> >> you intend for `ListLinks` to also be `!Unpin`, then you need a
> >> `PhantomPinned`)
> >
> > I need the `!Unpin` part for aliasing.
>
> Oh good point, do you mind adding a comment for that?
>
> >>> +}
> >>> +
> >>> +// SAFETY: The next/prev fields of a ListLinks can be moved across thread boundaries.
> >>
> >> Why? This is not a justification.
> >
> > What would you say?
>
> While trying to come up with a safety comment I thought about the
> following: this impl does not depend on the type that is behind the
> pointer (ie the type containing the `ListLinks`). Thus this `ListLinks`
> will always implement `Send` even if the pointed-to value does not.
> What we could do (and what definitely would be correct) is this:
> `List` can only be used with `Send` types, then we could implement
> `Send` for `ListLinks`. But I haven't actually come up with a problem,
> so there might a more permissive solution.
> Do you have a use-case where you need `!Send` types in a list?
>
> Here is a part of my reasoning: If the pointed-to value is `!Send`, then
> the `List` item type must also be `!Send`. Thus all list operations take
> place on the same thread (since the `List` will be `!Send`). Therefore
> nobody can access the `prev`/`next` pointers from another thread.
>
> But this does not justify that `ListLinks` can be made `Send`. (although
> there isn't actually a problem)

I don't think there's any reason to forbid lists with !Send types. The
List just becomes !Send too.

Alice