rust: add basic `Task`

It is an abstraction for C's `struct task_struct`. It implements
`AlwaysRefCounted`, so the refcount of the wrapped object is managed
safely on the Rust side.

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Link: https://lore.kernel.org/r/20230411054543.21278-9-wedsonaf@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/bindings/bindings_helper.h

@@ -8,6 +8,7 @@
 
 #include <linux/slab.h>
 #include <linux/refcount.h>
+#include <linux/sched.h>
 
 /* `bindgen` gets confused at certain things. */
 const gfp_t BINDINGS_GFP_KERNEL = GFP_KERNEL;

rust/helpers.c

@@ -24,6 +24,7 @@
 #include <linux/refcount.h>
 #include <linux/mutex.h>
 #include <linux/spinlock.h>
+#include <linux/sched/signal.h>
 
 __noreturn void rust_helper_BUG(void)
 {
@@ -60,6 +61,12 @@ void rust_helper_spin_unlock(spinlock_t *lock)
 }
 EXPORT_SYMBOL_GPL(rust_helper_spin_unlock);
 
+int rust_helper_signal_pending(struct task_struct *t)
+{
+	return signal_pending(t);
+}
+EXPORT_SYMBOL_GPL(rust_helper_signal_pending);
+
 refcount_t rust_helper_REFCOUNT_INIT(int n)
 {
 	return (refcount_t)REFCOUNT_INIT(n);
@@ -96,6 +103,18 @@ long rust_helper_PTR_ERR(__force const void *ptr)
 }
 EXPORT_SYMBOL_GPL(rust_helper_PTR_ERR);
 
+void rust_helper_get_task_struct(struct task_struct *t)
+{
+	get_task_struct(t);
+}
+EXPORT_SYMBOL_GPL(rust_helper_get_task_struct);
+
+void rust_helper_put_task_struct(struct task_struct *t)
+{
+	put_task_struct(t);
+}
+EXPORT_SYMBOL_GPL(rust_helper_put_task_struct);
+
 /*
  * We use `bindgen`'s `--size_t-is-usize` option to bind the C `size_t` type
  * as the Rust `usize` type, so we can use it in contexts where Rust

rust/kernel/lib.rs

@@ -44,6 +44,7 @@ mod static_assert;
 pub mod std_vendor;
 pub mod str;
 pub mod sync;
+pub mod task;
 pub mod types;
 
 #[doc(hidden)]

rust/kernel/task.rs

@@ -0,0 +1,75 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Tasks (threads and processes).
+//!
+//! C header: [`include/linux/sched.h`](../../../../include/linux/sched.h).
+
+use crate::{bindings, types::Opaque};
+use core::ptr;
+
+/// Wraps the kernel's `struct task_struct`.
+///
+/// # Invariants
+///
+/// All instances are valid tasks created by the C portion of the kernel.
+///
+/// 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`.
+#[repr(transparent)]
+pub struct Task(pub(crate) Opaque<bindings::task_struct>);
+
+// SAFETY: It's OK to access `Task` through references from other threads because we're either
+// accessing properties that don't change (e.g., `pid`, `group_leader`) or that are properly
+// synchronised by C code (e.g., `signal_pending`).
+unsafe impl Sync for Task {}
+
+/// The type of process identifiers (PIDs).
+type Pid = bindings::pid_t;
+
+impl Task {
+    /// 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
+        // have a valid group_leader.
+        let ptr = unsafe { *ptr::addr_of!((*self.0.get()).group_leader) };
+
+        // SAFETY: The lifetime of the returned task reference is tied to the lifetime of `self`,
+        // and given that a task has a reference to its group leader, we know it must be valid for
+        // the lifetime of the returned task reference.
+        unsafe { &*ptr.cast() }
+    }
+
+    /// Returns the PID of the given task.
+    pub fn pid(&self) -> Pid {
+        // SAFETY: By the type invariant, we know that `self.0` is a valid task. Valid tasks always
+        // have a valid pid.
+        unsafe { *ptr::addr_of!((*self.0.get()).pid) }
+    }
+
+    /// Determines whether the given task has pending signals.
+    pub fn signal_pending(&self) -> bool {
+        // SAFETY: By the type invariant, we know that `self.0` is valid.
+        unsafe { bindings::signal_pending(self.0.get()) != 0 }
+    }
+
+    /// Wakes up the task.
+    pub fn wake_up(&self) {
+        // SAFETY: By the type invariant, we know that `self.0.get()` is non-null and valid.
+        // And `wake_up_process` is safe to be called for any valid task, even if the task is
+        // running.
+        unsafe { bindings::wake_up_process(self.0.get()) };
+    }
+}
+
+// SAFETY: The type invariants guarantee that `Task` is always ref-counted.
+unsafe impl crate::types::AlwaysRefCounted for Task {
+    fn inc_ref(&self) {
+        // SAFETY: The existence of a shared reference means that the refcount is nonzero.
+        unsafe { bindings::get_task_struct(self.0.get()) };
+    }
+
+    unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
+        // SAFETY: The safety requirements guarantee that the refcount is nonzero.
+        unsafe { bindings::put_task_struct(obj.cast().as_ptr()) }
+    }
+}