Re: [PATCH 1/2] rust: sync: require `Send` and `Sync` for `Backend::State`

[Alice Ryhl]

On Tue, Sep 3, 2024 at 11:17 AM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
>
> `Lock<T, B>` implements `Send` and `Sync` when `T` is `Send` or `Sync`
> respectively. Since this does not depend on `B`, creating a `Lock` that
> is `Send` and `Sync`, but with a `!Sync` or `!Send` state is possible.
> This is a soundness issue, thus add the bounds to the respective impls.
>
> Signed-off-by: Benno Lossin <benno.lossin@xxxxxxxxx>

Currently, B::State is set directly to `bindings::spinlock_t` or
`bindings::mutex` and these types are pretty unlikely to be Send/Sync
on all kernel configurations. If you're going to make this change, you
will need to change these types.

Considering that B::State is already stored in Opaque meaning that we
don't run its destructor either, it's not really treated as a normal
field right now. Perhaps it would be better to change the safety
requirements of the `Backend` trait to impose restrictions on the
thread safety of B::State?

Alice