projects/linux·blob v6.7-rc8 · refs

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1// SPDX-License-Identifier: GPL-2.0
2
3//! Tasks (threads and processes).
4//!
5//! C header: [`include/linux/sched.h`](../../../../include/linux/sched.h).
6
7use crate::{bindings, types::Opaque};
8use core::{marker::PhantomData, ops::Deref, ptr};
9
10/// Returns the currently running task.
11#[macro_export]
12macro_rules! current {
13 () => {
14 // SAFETY: Deref + addr-of below create a temporary `TaskRef` that cannot outlive the
15 // caller.
16 unsafe { &*$crate::task::Task::current() }
17 };
18}
19
20/// Wraps the kernel's `struct task_struct`.
21///
22/// # Invariants
23///
24/// All instances are valid tasks created by the C portion of the kernel.
25///
26/// Instances of this type are always ref-counted, that is, a call to `get_task_struct` ensures
27/// that the allocation remains valid at least until the matching call to `put_task_struct`.
28///
29/// # Examples
30///
31/// The following is an example of getting the PID of the current thread with zero additional cost
32/// when compared to the C version:
33///
34/// ```
35/// let pid = current!().pid();
36/// ```
37///
38/// Getting the PID of the current process, also zero additional cost:
39///
40/// ```
41/// let pid = current!().group_leader().pid();
42/// ```
43///
44/// Getting the current task and storing it in some struct. The reference count is automatically
45/// incremented when creating `State` and decremented when it is dropped:
46///
47/// ```
48/// use kernel::{task::Task, types::ARef};
49///
50/// struct State {
51/// creator: ARef<Task>,
52/// index: u32,
53/// }
54///
55/// impl State {
56/// fn new() -> Self {
57/// Self {
58/// creator: current!().into(),
59/// index: 0,
60/// }
61/// }
62/// }
63/// ```
64#[repr(transparent)]
65pub struct Task(pub(crate) Opaque<bindings::task_struct>);
66
67// SAFETY: By design, the only way to access a `Task` is via the `current` function or via an
68// `ARef<Task>` obtained through the `AlwaysRefCounted` impl. This means that the only situation in
69// which a `Task` can be accessed mutably is when the refcount drops to zero and the destructor
70// runs. It is safe for that to happen on any thread, so it is ok for this type to be `Send`.
71unsafe impl Send for Task {}
72
73// SAFETY: It's OK to access `Task` through shared references from other threads because we're
74// either accessing properties that don't change (e.g., `pid`, `group_leader`) or that are properly
75// synchronised by C code (e.g., `signal_pending`).
76unsafe impl Sync for Task {}
77
78/// The type of process identifiers (PIDs).
79type Pid = bindings::pid_t;
80
81impl Task {
82 /// Returns a task reference for the currently executing task/thread.
83 ///
84 /// The recommended way to get the current task/thread is to use the
85 /// [`current`] macro because it is safe.
86 ///
87 /// # Safety
88 ///
89 /// Callers must ensure that the returned object doesn't outlive the current task/thread.
90 pub unsafe fn current() -> impl Deref<Target = Task> {
91 struct TaskRef<'a> {
92 task: &'a Task,
93 _not_send: PhantomData<*mut ()>,
94 }
95
96 impl Deref for TaskRef<'_> {
97 type Target = Task;
98
99 fn deref(&self) -> &Self::Target {
100 self.task
101 }
102 }
103
104 // SAFETY: Just an FFI call with no additional safety requirements.
105 let ptr = unsafe { bindings::get_current() };
106
107 TaskRef {
108 // SAFETY: If the current thread is still running, the current task is valid. Given
109 // that `TaskRef` is not `Send`, we know it cannot be transferred to another thread
110 // (where it could potentially outlive the caller).
111 task: unsafe { &*ptr.cast() },
112 _not_send: PhantomData,
113 }
114 }
115
116 /// Returns the group leader of the given task.
117 pub fn group_leader(&self) -> &Task {
118 // SAFETY: By the type invariant, we know that `self.0` is a valid task. Valid tasks always
119 // have a valid group_leader.
120 let ptr = unsafe { *ptr::addr_of!((*self.0.get()).group_leader) };
121
122 // SAFETY: The lifetime of the returned task reference is tied to the lifetime of `self`,
123 // and given that a task has a reference to its group leader, we know it must be valid for
124 // the lifetime of the returned task reference.
125 unsafe { &*ptr.cast() }
126 }
127
128 /// Returns the PID of the given task.
129 pub fn pid(&self) -> Pid {
130 // SAFETY: By the type invariant, we know that `self.0` is a valid task. Valid tasks always
131 // have a valid pid.
132 unsafe { *ptr::addr_of!((*self.0.get()).pid) }
133 }
134
135 /// Determines whether the given task has pending signals.
136 pub fn signal_pending(&self) -> bool {
137 // SAFETY: By the type invariant, we know that `self.0` is valid.
138 unsafe { bindings::signal_pending(self.0.get()) != 0 }
139 }
140
141 /// Wakes up the task.
142 pub fn wake_up(&self) {
143 // SAFETY: By the type invariant, we know that `self.0.get()` is non-null and valid.
144 // And `wake_up_process` is safe to be called for any valid task, even if the task is
145 // running.
146 unsafe { bindings::wake_up_process(self.0.get()) };
147 }
148}
149
150// SAFETY: The type invariants guarantee that `Task` is always ref-counted.
151unsafe impl crate::types::AlwaysRefCounted for Task {
152 fn inc_ref(&self) {
153 // SAFETY: The existence of a shared reference means that the refcount is nonzero.
154 unsafe { bindings::get_task_struct(self.0.get()) };
155 }
156
157 unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
158 // SAFETY: The safety requirements guarantee that the refcount is nonzero.
159 unsafe { bindings::put_task_struct(obj.cast().as_ptr()) }
160 }
161}