Re: [PATCH 10/13] rust: introduce `Task::current`

From: Alice Ryhl
Date: Fri Mar 31 2023 - 03:32:43 EST

On 3/31/23 04:47, Gary Guo wrote:
On Thu, 30 Mar 2023 01:39:51 -0300
Wedson Almeida Filho <wedsonaf@xxxxxxxxx> wrote:

From: Wedson Almeida Filho <walmeida@xxxxxxxxxxxxx>

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@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Wedson Almeida Filho <walmeida@xxxxxxxxxxxxx>
---
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,
+ }
+ }
+

I don't think this API is sound, as you can do `&*Task::current()` and
get a `&'static Task`, which is very problematic.

A sound API would be

pub fn with_current<R>(f: imp FnOnce(&Task) -> R) -> R { ... }

(which also is how thread local works in Rust)

You would have to write `Task::with_current(|cur| cur.pid())` though,
which unfortunately is a bit less ergonomic.

Best,
Gary

This is true, unfortunately. It would be possible to write a macro with a more similar API to the current implementation.