[PATCH v3 10/13] rust: introduce `current`

Wedson Almeida Filho
2023-04-08T07:55:22Z

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>
---
v1 -> v2: Make `current` a macro to prevent it from escaping the caller
v2 -> v3:
- Mention `current` macro in `Task::current`
- Hide implementation of `TaskRef` inside `Task::current`

rust/helpers.c | 6 +++
rust/kernel/prelude.rs | 2 +
rust/kernel/task.rs | 88 +++++++++++++++++++++++++++++++++++++++++-
3 files changed, 95 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/prelude.rs b/rust/kernel/prelude.rs
index fcdc511d2ce8..c28587d68ebc 100644
--- a/rust/kernel/prelude.rs
+++ b/rust/kernel/prelude.rs
@@ -36,3 +36,5 @@ pub use super::error::{code::*, Error, Result};
pub use super::{str::CStr, ThisModule};

pub use super::init::{InPlaceInit, Init, PinInit};
+
+pub use super::current;
diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
index d70cad131956..5269a562cb1b 100644
--- a/rust/kernel/task.rs
+++ b/rust/kernel/task.rs
@@ -5,7 +5,17 @@
//! C header: [`include/linux/sched.h`](../../../../include/linux/sched.h).

use crate::{bindings, types::Opaque};
-use core::ptr;
+use core::{marker::PhantomData, ops::Deref, ptr};
+
+/// Returns the currently running task.
+#[macro_export]
+macro_rules! current {
+ () => {
+ // SAFETY: Deref + addr-of below create a temporary `TaskRef` that cannot outlive the
+ // caller.
+ unsafe { &*$crate::task::Task::current() }
+ };
+}

/// Wraps the kernel's `struct task_struct`.
///
@@ -15,6 +25,42 @@ use core::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:
+///
+/// ```
+/// let pid = current!().pid();
+/// ```
+///
+/// Getting the PID of the current process, also zero additional cost:
+///
+/// ```
+/// let pid = 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, types::ARef};
+///
+/// struct State {
+/// creator: ARef<Task>,
+/// index: u32,
+/// }
+///
+/// impl State {
+/// fn new() -> Self {
+/// Self {
+/// creator: current!().into(),
+/// index: 0,
+/// }
+/// }
+/// }
+/// ```
#[repr(transparent)]
pub struct Task(pub(crate) Opaque<bindings::task_struct>);

@@ -27,6 +73,46 @@ unsafe impl Sync for Task {}
type Pid = bindings::pid_t;

impl Task {
+ /// Returns a task reference for the currently executing task/thread.
+ ///
+ /// The recommended way to get the current task/thread is to use the
+ /// [`current`](crate::current) macro because it is safe.
+ ///
+ /// # Safety
+ ///
+ /// Callers must ensure that the returned object doesn't outlive the current task/thread.
+ pub unsafe fn current() -> impl Deref<Target = Task> {
+ 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()
+ }
+ }
+
+ // 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 a valid task. Valid tasks always
--
2.34.1