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![Alice Ryhl](/assets/img/avatar_default.png)
When a type also implements `Sync`, the meaning of `Send` is just "this type may be accessed mutably from threads other than the one it is created on". That's ok for this type. Signed-off-by: Alice Ryhl <aliceryhl@google.com> Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com> Reviewed-by: Gary Guo <gary@garyguo.net> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com> Reviewed-by: Benno Lossin <benno.lossin@proton.me> Link: https://lore.kernel.org/r/20230531145939.3714886-5-aliceryhl@google.com Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
161 lines
5.6 KiB
Rust
161 lines
5.6 KiB
Rust
// SPDX-License-Identifier: GPL-2.0
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//! Tasks (threads and processes).
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//!
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//! C header: [`include/linux/sched.h`](../../../../include/linux/sched.h).
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use crate::{bindings, types::Opaque};
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use core::{marker::PhantomData, ops::Deref, ptr};
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/// Returns the currently running task.
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#[macro_export]
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macro_rules! current {
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() => {
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// SAFETY: Deref + addr-of below create a temporary `TaskRef` that cannot outlive the
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// caller.
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unsafe { &*$crate::task::Task::current() }
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};
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}
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/// Wraps the kernel's `struct task_struct`.
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///
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/// # Invariants
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///
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/// All instances are valid tasks created by the C portion of the kernel.
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///
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/// Instances of this type are always ref-counted, that is, a call to `get_task_struct` ensures
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/// that the allocation remains valid at least until the matching call to `put_task_struct`.
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///
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/// # Examples
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///
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/// The following is an example of getting the PID of the current thread with zero additional cost
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/// when compared to the C version:
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///
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/// ```
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/// let pid = current!().pid();
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/// ```
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///
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/// Getting the PID of the current process, also zero additional cost:
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///
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/// ```
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/// let pid = current!().group_leader().pid();
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/// ```
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///
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/// Getting the current task and storing it in some struct. The reference count is automatically
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/// incremented when creating `State` and decremented when it is dropped:
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///
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/// ```
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/// use kernel::{task::Task, types::ARef};
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///
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/// struct State {
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/// creator: ARef<Task>,
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/// index: u32,
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/// }
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///
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/// impl State {
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/// fn new() -> Self {
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/// Self {
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/// creator: current!().into(),
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/// index: 0,
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/// }
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/// }
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/// }
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/// ```
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#[repr(transparent)]
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pub struct Task(pub(crate) Opaque<bindings::task_struct>);
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// SAFETY: By design, the only way to access a `Task` is via the `current` function or via an
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// `ARef<Task>` obtained through the `AlwaysRefCounted` impl. This means that the only situation in
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// which a `Task` can be accessed mutably is when the refcount drops to zero and the destructor
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// runs. It is safe for that to happen on any thread, so it is ok for this type to be `Send`.
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unsafe impl Send for Task {}
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// SAFETY: It's OK to access `Task` through shared references from other threads because we're
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// either accessing properties that don't change (e.g., `pid`, `group_leader`) or that are properly
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// synchronised by C code (e.g., `signal_pending`).
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unsafe impl Sync for Task {}
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/// The type of process identifiers (PIDs).
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type Pid = bindings::pid_t;
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impl Task {
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/// Returns a task reference for the currently executing task/thread.
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///
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/// The recommended way to get the current task/thread is to use the
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/// [`current`](crate::current) macro because it is safe.
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///
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/// # Safety
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///
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/// Callers must ensure that the returned object doesn't outlive the current task/thread.
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pub unsafe fn current() -> impl Deref<Target = Task> {
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struct TaskRef<'a> {
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task: &'a Task,
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_not_send: PhantomData<*mut ()>,
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}
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impl Deref for TaskRef<'_> {
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type Target = Task;
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fn deref(&self) -> &Self::Target {
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self.task
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}
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}
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// SAFETY: Just an FFI call with no additional safety requirements.
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let ptr = unsafe { bindings::get_current() };
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TaskRef {
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// SAFETY: If the current thread is still running, the current task is valid. Given
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// that `TaskRef` is not `Send`, we know it cannot be transferred to another thread
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// (where it could potentially outlive the caller).
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task: unsafe { &*ptr.cast() },
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_not_send: PhantomData,
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}
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}
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/// Returns the group leader of the given task.
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pub fn group_leader(&self) -> &Task {
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// SAFETY: By the type invariant, we know that `self.0` is a valid task. Valid tasks always
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// have a valid group_leader.
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let ptr = unsafe { *ptr::addr_of!((*self.0.get()).group_leader) };
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// SAFETY: The lifetime of the returned task reference is tied to the lifetime of `self`,
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// and given that a task has a reference to its group leader, we know it must be valid for
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// the lifetime of the returned task reference.
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unsafe { &*ptr.cast() }
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}
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/// Returns the PID of the given task.
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pub fn pid(&self) -> Pid {
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// SAFETY: By the type invariant, we know that `self.0` is a valid task. Valid tasks always
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// have a valid pid.
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unsafe { *ptr::addr_of!((*self.0.get()).pid) }
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}
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/// Determines whether the given task has pending signals.
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pub fn signal_pending(&self) -> bool {
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// SAFETY: By the type invariant, we know that `self.0` is valid.
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unsafe { bindings::signal_pending(self.0.get()) != 0 }
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}
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/// Wakes up the task.
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pub fn wake_up(&self) {
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// SAFETY: By the type invariant, we know that `self.0.get()` is non-null and valid.
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// And `wake_up_process` is safe to be called for any valid task, even if the task is
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// running.
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unsafe { bindings::wake_up_process(self.0.get()) };
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}
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}
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// SAFETY: The type invariants guarantee that `Task` is always ref-counted.
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unsafe impl crate::types::AlwaysRefCounted for Task {
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fn inc_ref(&self) {
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// SAFETY: The existence of a shared reference means that the refcount is nonzero.
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unsafe { bindings::get_task_struct(self.0.get()) };
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}
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unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
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// SAFETY: The safety requirements guarantee that the refcount is nonzero.
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unsafe { bindings::put_task_struct(obj.cast().as_ptr()) }
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}
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}
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