> 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@xxxxxxxxx> > Reviewed-by: Benno Lossin <benno.lossin@xxxxxxxxx> > Reviewed-by: Trevor Gross <tmgross@xxxxxxxxx> > Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@xxxxxxxxx> > Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx> > --- > 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@xxxxxxxxxxxxxx> 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
