linux-kernel - Re: [PATCH v4 1/9] rust: types: add `NotThreadSafe`

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Date: Mon, 5 Feb 2024 21:14:15 -0500
From: Trevor Gross <tmgross@...ch.edu>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.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>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...sung.com>, 
	Peter Zijlstra <peterz@...radead.org>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Arve Hjønnevåg <arve@...roid.com>, 
	Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>, 
	Joel Fernandes <joel@...lfernandes.org>, Carlos Llamas <cmllamas@...gle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Dan Williams <dan.j.williams@...el.com>, 
	Kees Cook <keescook@...omium.org>, Matthew Wilcox <willy@...radead.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Daniel Xu <dxu@...uu.xyz>, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v4 1/9] rust: types: add `NotThreadSafe`

On Fri, Feb 2, 2024 at 5:56 AM Alice Ryhl <aliceryhl@...gle.com> wrote:
>
> This introduces a new marker type for types that shouldn't be thread
> safe. By adding a field of this type to a struct, it becomes non-Send
> and non-Sync, which means that it cannot be accessed in any way from
> threads other than the one it was created on.
>
> This is useful for APIs that require globals such as `current` to remain
> constant while the value exists.
>
> We update two existing users in the Kernel to use this helper:
>
>  * `Task::current()` - moving the return type of this value to a
>    different thread would not be safe as you can no longer be guaranteed
>    that the `current` pointer remains valid.
>  * Lock guards. Mutexes and spinlocks should be unlocked on the same
>    thread as where they were locked, so we enforce this using the Send
>    trait.
>
> There are also additional users in later patches of this patchset. See
> [1] and [2] for the discussion that led to the introducion of this

s/introducion/introduction

> patch.
>
> Link: https://lore.kernel.org/all/nFDPJFnzE9Q5cqY7FwSMByRH2OAn_BpI4H53NQfWIlN6I2qfmAqnkp2wRqn0XjMO65OyZY4h6P4K2nAGKJpAOSzksYXaiAK_FoH_8QbgBI4=@proton.me/ [1]
> Link: https://lore.kernel.org/all/nFDPJFnzE9Q5cqY7FwSMByRH2OAn_BpI4H53NQfWIlN6I2qfmAqnkp2wRqn0XjMO65OyZY4h6P4K2nAGKJpAOSzksYXaiAK_FoH_8QbgBI4=@proton.me/ [2]
> Suggested-by: Benno Lossin <benno.lossin@...ton.me>
> Reviewed-by: Benno Lossin <benno.lossin@...ton.me>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>  rust/kernel/sync/lock.rs | 15 +++++++++++----
>  rust/kernel/task.rs      | 10 ++++++----
>  rust/kernel/types.rs     | 17 +++++++++++++++++
>  3 files changed, 34 insertions(+), 8 deletions(-)
>
> diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> index 149a5259d431..d08d9d32915a 100644
> --- a/rust/kernel/sync/lock.rs
> +++ b/rust/kernel/sync/lock.rs
> @@ -6,8 +6,15 @@
>  //! spinlocks, raw spinlocks) to be provided with minimal effort.
>
>  use super::LockClassKey;
> -use crate::{bindings, init::PinInit, pin_init, str::CStr, types::Opaque, types::ScopeGuard};
> -use core::{cell::UnsafeCell, marker::PhantomData, marker::PhantomPinned};
> +use crate::{
> +    bindings,
> +    init::PinInit,
> +    pin_init,
> +    str::CStr,
> +    types::ScopeGuard,
> +    types::{NotThreadSafe, Opaque},

Formatting nit: ScopeGuard could probably be placed in the same group
as NotThreadSafe & Opaque

> [...]
> +
> +/// Zero-sized type to mark types not [`Send`].
> +///
> +/// Add this type as a field to your struct if your type should not be sent to a different task.
> +/// Since [`Send`] is an auto trait, adding a single field that is `!Send` will ensure that the
> +/// whole type is `!Send`.
> +///
> +/// If a type is `!Send` it is impossible to give control over an instance of the type to another
> +/// task. This is useful when a type stores task-local information for example file descriptors.

I initially read this thinking it meant to include this type if your
struct also had a FD rather than being part of the FD. Maybe

    This is useful to include in types that store or reference task-local
    information. A file descriptor is an example of one such type.

> +pub type NotThreadSafe = PhantomData<*mut ()>;
> +
> +/// Used to construct instances of type [`NotThreadSafe`] similar to how we construct
> +/// `PhantomData`.

I think it sounds slightly better reworded from personal to passive, i.e.

    ... similar to how `PhantomData` is constructed.

> +/// [`NotThreadSafe`]: type@...ThreadSafe
> +#[allow(non_upper_case_globals)]
> +pub const NotThreadSafe: NotThreadSafe = PhantomData;
> --
> 2.43.0.594.gd9cf4e227d-goog

This looks good, sounds nice to make the intent more clear. Nothing
that isn't optional, so

Reviewed-by: Trevor Gross <tmgross@...ch.edu>