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Date: Thu, 04 Jul 2024 15:55:37 +0000
From: Björn Roy Baron <bjorn3_gh@...tonmail.com>
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>, 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, Trevor Gross <tmgross@...ch.edu>, Martin Rodriguez
	Reboredo <yakoyoku@...il.com>
Subject: Re: [PATCH v7 1/8] rust: types: add `NotThreadSafe`

> 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 introduction of this
> 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>
> Reviewed-by: Trevor Gross <tmgross@...ch.edu>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@...il.com>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>  rust/kernel/sync/lock.rs | 13 +++++++++----
>  rust/kernel/task.rs      | 10 ++++++----
>  rust/kernel/types.rs     | 18 ++++++++++++++++++
>  3 files changed, 33 insertions(+), 8 deletions(-)
> 
> diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> index f6c34ca4d819..d6e9bab114b8 100644
> --- a/rust/kernel/sync/lock.rs
> +++ b/rust/kernel/sync/lock.rs
> @@ -6,8 +6,13 @@
>  //! spinlocks, raw spinlocks) to be provided with minimal effort.
> 
>  use super::LockClassKey;
> -use crate::{init::PinInit, pin_init, str::CStr, types::Opaque, types::ScopeGuard};
> -use core::{cell::UnsafeCell, marker::PhantomData, marker::PhantomPinned};
> +use crate::{
> +    init::PinInit,
> +    pin_init,
> +    str::CStr,
> +    types::{NotThreadSafe, Opaque, ScopeGuard},
> +};
> +use core::{cell::UnsafeCell, marker::PhantomPinned};
>  use macros::pin_data;
> 
>  pub mod mutex;
> @@ -139,7 +144,7 @@ pub fn lock(&self) -> Guard<'_, T, B> {
>  pub struct Guard<'a, T: ?Sized, B: Backend> {
>      pub(crate) lock: &'a Lock<T, B>,
>      pub(crate) state: B::GuardState,
> -    _not_send: PhantomData<*mut ()>,
> +    _not_send: NotThreadSafe,
>  }
> 
>  // SAFETY: `Guard` is sync when the data protected by the lock is also sync.
> @@ -191,7 +196,7 @@ pub(crate) unsafe fn new(lock: &'a Lock<T, B>, state: B::GuardState) -> Self {
>          Self {
>              lock,
>              state,
> -            _not_send: PhantomData,
> +            _not_send: NotThreadSafe,
>          }
>      }
>  }
> diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
> index 55dff7e088bf..278c623de0c6 100644
> --- a/rust/kernel/task.rs
> +++ b/rust/kernel/task.rs
> @@ -4,10 +4,12 @@
>  //!
>  //! C header: [`include/linux/sched.h`](srctree/include/linux/sched.h).
> 
> -use crate::types::Opaque;
> +use crate::{
> +    bindings,
> +    types::{NotThreadSafe, Opaque},
> +};
>  use core::{
>      ffi::{c_int, c_long, c_uint},
> -    marker::PhantomData,
>      ops::Deref,
>      ptr,
>  };
> @@ -106,7 +108,7 @@ impl Task {
>      pub unsafe fn current() -> impl Deref<Target = Task> {
>          struct TaskRef<'a> {
>              task: &'a Task,
> -            _not_send: PhantomData<*mut ()>,
> +            _not_send: NotThreadSafe,
>          }
> 
>          impl Deref for TaskRef<'_> {
> @@ -125,7 +127,7 @@ fn deref(&self) -> &Self::Target {
>              // that `TaskRef` is not `Send`, we know it cannot be transferred to another thread
>              // (where it could potentially outlive the caller).
>              task: unsafe { &*ptr.cast() },
> -            _not_send: PhantomData,
> +            _not_send: NotThreadSafe,
>          }
>      }
> 
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 2e7c9008621f..93734677cfe7 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -409,3 +409,21 @@ pub enum Either<L, R> {
>      /// Constructs an instance of [`Either`] containing a value of type `R`.
>      Right(R),
>  }
> +
> +/// 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 to include in types that store or reference task-local information. A file
> +/// descriptor is an example of such task-local information.
> +pub type NotThreadSafe = PhantomData<*mut ()>;

This also makes the type not Sync. For mutex guards it should be fine to
be Sync if the wrapped type is Sync. Mutex guards not being Sync is
pre-existing, so not something to fix in this patch. NotThreadSafe should
probably document that it also markes a type as not Sync though.

(For those not familiar with the difference, Send indicates that ownership
of a value can be transferred to another thread, while Sync indicates that
a value can be accessed from another thread through a shared reference,
but not necessarily dropped from said thread.)

Having a dedicated type for this makes things clearer.

Reviewed-by: Björn Roy Baron <bjorn3_gh@...tonmail.com>
with the docs updated to mention Sync.

> +
> +/// Used to construct instances of type [`NotThreadSafe`] similar to how `PhantomData` is
> +/// constructed.
> +///
> +/// [`NotThreadSafe`]: type@...ThreadSafe
> +#[allow(non_upper_case_globals)]
> +pub const NotThreadSafe: NotThreadSafe = PhantomData;
> 
> --
> 2.45.2.803.g4e1b14247a-goog

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