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