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    Subject[PATCH 10/13] rust: introduce `Task::current`
    Date
    From: Wedson Almeida Filho <walmeida@microsoft.com>

    This allows Rust code to get a reference to the current task without
    having to increment the refcount, but still guaranteeing memory safety.

    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
    ---
    rust/helpers.c | 6 ++++
    rust/kernel/task.rs | 83 ++++++++++++++++++++++++++++++++++++++++++++-
    2 files changed, 88 insertions(+), 1 deletion(-)

    diff --git a/rust/helpers.c b/rust/helpers.c
    index 58a194042c86..96441744030e 100644
    --- a/rust/helpers.c
    +++ b/rust/helpers.c
    @@ -100,6 +100,12 @@ bool rust_helper_refcount_dec_and_test(refcount_t *r)
    }
    EXPORT_SYMBOL_GPL(rust_helper_refcount_dec_and_test);

    +struct task_struct *rust_helper_get_current(void)
    +{
    + return current;
    +}
    +EXPORT_SYMBOL_GPL(rust_helper_get_current);
    +
    void rust_helper_get_task_struct(struct task_struct *t)
    {
    get_task_struct(t);
    diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
    index 8d7a8222990f..8b2b56ba9c6d 100644
    --- a/rust/kernel/task.rs
    +++ b/rust/kernel/task.rs
    @@ -5,7 +5,7 @@
    //! C header: [`include/linux/sched.h`](../../../../include/linux/sched.h).

    use crate::bindings;
    -use core::{cell::UnsafeCell, ptr};
    +use core::{cell::UnsafeCell, marker::PhantomData, ops::Deref, ptr};

    /// Wraps the kernel's `struct task_struct`.
    ///
    @@ -13,6 +13,46 @@ use core::{cell::UnsafeCell, ptr};
    ///
    /// Instances of this type are always ref-counted, that is, a call to `get_task_struct` ensures
    /// that the allocation remains valid at least until the matching call to `put_task_struct`.
    +///
    +/// # Examples
    +///
    +/// The following is an example of getting the PID of the current thread with zero additional cost
    +/// when compared to the C version:
    +///
    +/// ```
    +/// use kernel::task::Task;
    +///
    +/// let pid = Task::current().pid();
    +/// ```
    +///
    +/// Getting the PID of the current process, also zero additional cost:
    +///
    +/// ```
    +/// use kernel::task::Task;
    +///
    +/// let pid = Task::current().group_leader().pid();
    +/// ```
    +///
    +/// Getting the current task and storing it in some struct. The reference count is automatically
    +/// incremented when creating `State` and decremented when it is dropped:
    +///
    +/// ```
    +/// use kernel::{task::Task, ARef};
    +///
    +/// struct State {
    +/// creator: ARef<Task>,
    +/// index: u32,
    +/// }
    +///
    +/// impl State {
    +/// fn new() -> Self {
    +/// Self {
    +/// creator: Task::current().into(),
    +/// index: 0,
    +/// }
    +/// }
    +/// }
    +/// ```
    #[repr(transparent)]
    pub struct Task(pub(crate) UnsafeCell<bindings::task_struct>);

    @@ -25,6 +65,20 @@ unsafe impl Sync for Task {}
    type Pid = bindings::pid_t;

    impl Task {
    + /// Returns a task reference for the currently executing task/thread.
    + pub fn current<'a>() -> TaskRef<'a> {
    + // SAFETY: Just an FFI call with no additional safety requirements.
    + let ptr = unsafe { bindings::get_current() };
    +
    + TaskRef {
    + // SAFETY: If the current thread is still running, the current task is valid. Given
    + // 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,
    + }
    + }
    +
    /// Returns the group leader of the given task.
    pub fn group_leader(&self) -> &Task {
    // SAFETY: By the type invariant, we know that `self.0` is valid.
    @@ -69,3 +123,30 @@ unsafe impl crate::types::AlwaysRefCounted for Task {
    unsafe { bindings::put_task_struct(obj.cast().as_ptr()) }
    }
    }
    +
    +/// A wrapper for a shared reference to [`Task`] that isn't [`Send`].
    +///
    +/// We make this explicitly not [`Send`] so that we can use it to represent the current thread
    +/// without having to increment/decrement the task's reference count.
    +///
    +/// # Invariants
    +///
    +/// The wrapped [`Task`] remains valid for the lifetime of the object.
    +pub struct TaskRef<'a> {
    + task: &'a Task,
    + _not_send: PhantomData<*mut ()>,
    +}
    +
    +impl Deref for TaskRef<'_> {
    + type Target = Task;
    +
    + fn deref(&self) -> &Self::Target {
    + self.task
    + }
    +}
    +
    +impl From<TaskRef<'_>> for crate::types::ARef<Task> {
    + fn from(t: TaskRef<'_>) -> Self {
    + t.deref().into()
    + }
    +}
    --
    2.34.1
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