1 // SPDX-License-Identifier: GPL-2.0
2
3 //! Tasks (threads and processes).
4 //!
5 //! C header: [`include/linux/sched.h`](srctree/include/linux/sched.h).
6
7 use crate::types::Opaque;
8 use core::{
9 ffi::{c_int, c_long, c_uint},
10 marker::PhantomData,
11 ops::Deref,
12 ptr,
13 };
14
15 /// A sentinel value used for infinite timeouts.
16 pub const MAX_SCHEDULE_TIMEOUT: c_long = c_long::MAX;
17
18 /// Bitmask for tasks that are sleeping in an interruptible state.
19 pub const TASK_INTERRUPTIBLE: c_int = bindings::TASK_INTERRUPTIBLE as c_int;
20 /// Bitmask for tasks that are sleeping in an uninterruptible state.
21 pub const TASK_UNINTERRUPTIBLE: c_int = bindings::TASK_UNINTERRUPTIBLE as c_int;
22 /// Convenience constant for waking up tasks regardless of whether they are in interruptible or
23 /// uninterruptible sleep.
24 pub const TASK_NORMAL: c_uint = bindings::TASK_NORMAL as c_uint;
25
26 /// Returns the currently running task.
27 #[macro_export]
28 macro_rules! current {
29 () => {
30 // SAFETY: Deref + addr-of below create a temporary `TaskRef` that cannot outlive the
31 // caller.
32 unsafe { &*$crate::task::Task::current() }
33 };
34 }
35
36 /// Wraps the kernel's `struct task_struct`.
37 ///
38 /// # Invariants
39 ///
40 /// All instances are valid tasks created by the C portion of the kernel.
41 ///
42 /// Instances of this type are always refcounted, that is, a call to `get_task_struct` ensures
43 /// that the allocation remains valid at least until the matching call to `put_task_struct`.
44 ///
45 /// # Examples
46 ///
47 /// The following is an example of getting the PID of the current thread with zero additional cost
48 /// when compared to the C version:
49 ///
50 /// ```
51 /// let pid = current!().pid();
52 /// ```
53 ///
54 /// Getting the PID of the current process, also zero additional cost:
55 ///
56 /// ```
57 /// let pid = current!().group_leader().pid();
58 /// ```
59 ///
60 /// Getting the current task and storing it in some struct. The reference count is automatically
61 /// incremented when creating `State` and decremented when it is dropped:
62 ///
63 /// ```
64 /// use kernel::{task::Task, types::ARef};
65 ///
66 /// struct State {
67 /// creator: ARef<Task>,
68 /// index: u32,
69 /// }
70 ///
71 /// impl State {
72 /// fn new() -> Self {
73 /// Self {
74 /// creator: current!().into(),
75 /// index: 0,
76 /// }
77 /// }
78 /// }
79 /// ```
80 #[repr(transparent)]
81 pub struct Task(pub(crate) Opaque<bindings::task_struct>);
82
83 // SAFETY: By design, the only way to access a `Task` is via the `current` function or via an
84 // `ARef<Task>` obtained through the `AlwaysRefCounted` impl. This means that the only situation in
85 // which a `Task` can be accessed mutably is when the refcount drops to zero and the destructor
86 // runs. It is safe for that to happen on any thread, so it is ok for this type to be `Send`.
87 unsafe impl Send for Task {}
88
89 // SAFETY: It's OK to access `Task` through shared references from other threads because we're
90 // either accessing properties that don't change (e.g., `pid`, `group_leader`) or that are properly
91 // synchronised by C code (e.g., `signal_pending`).
92 unsafe impl Sync for Task {}
93
94 /// The type of process identifiers (PIDs).
95 type Pid = bindings::pid_t;
96
97 impl Task {
98 /// Returns a task reference for the currently executing task/thread.
99 ///
100 /// The recommended way to get the current task/thread is to use the
101 /// [`current`] macro because it is safe.
102 ///
103 /// # Safety
104 ///
105 /// Callers must ensure that the returned object doesn't outlive the current task/thread.
106 pub unsafe fn current() -> impl Deref<Target = Task> {
107 struct TaskRef<'a> {
108 task: &'a Task,
109 _not_send: PhantomData<*mut ()>,
110 }
111
112 impl Deref for TaskRef<'_> {
113 type Target = Task;
114
115 fn deref(&self) -> &Self::Target {
116 self.task
117 }
118 }
119
120 // SAFETY: Just an FFI call with no additional safety requirements.
121 let ptr = unsafe { bindings::get_current() };
122
123 TaskRef {
124 // SAFETY: If the current thread is still running, the current task is valid. Given
125 // that `TaskRef` is not `Send`, we know it cannot be transferred to another thread
126 // (where it could potentially outlive the caller).
127 task: unsafe { &*ptr.cast() },
128 _not_send: PhantomData,
129 }
130 }
131
132 /// Returns the group leader of the given task.
133 pub fn group_leader(&self) -> &Task {
134 // SAFETY: By the type invariant, we know that `self.0` is a valid task. Valid tasks always
135 // have a valid `group_leader`.
136 let ptr = unsafe { *ptr::addr_of!((*self.0.get()).group_leader) };
137
138 // SAFETY: The lifetime of the returned task reference is tied to the lifetime of `self`,
139 // and given that a task has a reference to its group leader, we know it must be valid for
140 // the lifetime of the returned task reference.
141 unsafe { &*ptr.cast() }
142 }
143
144 /// Returns the PID of the given task.
145 pub fn pid(&self) -> Pid {
146 // SAFETY: By the type invariant, we know that `self.0` is a valid task. Valid tasks always
147 // have a valid pid.
148 unsafe { *ptr::addr_of!((*self.0.get()).pid) }
149 }
150
151 /// Determines whether the given task has pending signals.
152 pub fn signal_pending(&self) -> bool {
153 // SAFETY: By the type invariant, we know that `self.0` is valid.
154 unsafe { bindings::signal_pending(self.0.get()) != 0 }
155 }
156
157 /// Wakes up the task.
158 pub fn wake_up(&self) {
159 // SAFETY: By the type invariant, we know that `self.0.get()` is non-null and valid.
160 // And `wake_up_process` is safe to be called for any valid task, even if the task is
161 // running.
162 unsafe { bindings::wake_up_process(self.0.get()) };
163 }
164 }
165
166 // SAFETY: The type invariants guarantee that `Task` is always refcounted.
167 unsafe impl crate::types::AlwaysRefCounted for Task {
168 fn inc_ref(&self) {
169 // SAFETY: The existence of a shared reference means that the refcount is nonzero.
170 unsafe { bindings::get_task_struct(self.0.get()) };
171 }
172
173 unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
174 // SAFETY: The safety requirements guarantee that the refcount is nonzero.
175 unsafe { bindings::put_task_struct(obj.cast().as_ptr()) }
176 }
177 }
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