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@proton.me>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.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 ()>;
+
+/// Used to construct instances of type [`NotThreadSafe`] similar to how `PhantomData` is
+/// constructed.
+///
+/// [`NotThreadSafe`]: type@NotThreadSafe
+#[allow(non_upper_case_globals)]
+pub const NotThreadSafe: NotThreadSafe = PhantomData;

-- 
2.45.2.803.g4e1b14247a-goog

> 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@proton.me>
> Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> Reviewed-by: Trevor Gross <tmgross@umich.edu>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.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@protonmail.com>
with the docs updated to mention Sync.

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