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

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Message-ID: <CAH5fLgihM6OeuVrAhSXAqb9RDKBa8p7+EOsFuuqkGu-OzoapZw@mail.gmail.com>
Date: Tue, 3 Sep 2024 11:30:14 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...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>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] rust: sync: require `Send` and `Sync` for `Backend::State`

On Tue, Sep 3, 2024 at 11:17 AM Benno Lossin <benno.lossin@...ton.me> 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@...ton.me>

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