Messages in this thread | | | Date | Mon, 10 Apr 2023 18:04:51 +0000 | From | Benno Lossin <> | Subject | Re: [PATCH v3 10/13] rust: introduce `current` |
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On 08.04.23 09:53, Wedson Almeida Filho wrote: > 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() > + } > + }
I think we can remove this `From` impl, since the type is never exposed to the outside (there still is the `From<&T> for ARef<T>` impl, so users can still do `current!().into()` to get an `ARef<Task>`).
-- Cheers, Benno
> + > + // 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 >
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