new: lazy_init (#230)

kernel/src/libs/lazy_init.rs

@@ -0,0 +1,159 @@
+// Copyright (C) DragonOS Community  longjin
+
+// This program is free software; you can redistribute it and/or
+// modify it under the terms of the GNU General Public License
+// as published by the Free Software Foundation; either version 2
+// of the License, or (at your option) any later version.
+
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with this program; if not, write to the Free Software
+// Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+// Or you can visit https://www.gnu.org/licenses/gpl-2.0.html
+#![allow(dead_code)]
+
+use core::cell::UnsafeCell;
+use core::fmt::Debug;
+use core::mem::MaybeUninit;
+use core::ops::{Deref, DerefMut};
+use core::sync::atomic::{AtomicBool, Ordering};
+
+use super::spinlock::SpinLock;
+
+/// A wrapper around a value that is initialized lazily.
+/// The value is initialized the first time it is accessed.
+/// This is useful for initializing values that are expensive to initialize.
+/// The value is initialized using the provided closure.
+/// The closure is only called once, and the result is cached.
+/// The value is not initialized if it is never accessed.
+pub struct Lazy<T: Sync> {
+    /// The lock that is used to ensure that only one thread calls the init function at the same time.
+    init_lock: SpinLock<()>,
+    /// The value that is initialized lazily.
+    value: UnsafeCell<MaybeUninit<T>>,
+    /// Whether the value has been initialized.
+    initialized: AtomicBool,
+}
+
+impl<T: Sync> Lazy<T> {
+    /// Creates a new `Lazy` value that will be initialized with the
+    /// result of the closure `init`.
+    pub const fn new() -> Lazy<T> {
+        Lazy {
+            value: UnsafeCell::new(MaybeUninit::uninit()),
+            initialized: AtomicBool::new(false),
+            init_lock: SpinLock::new(()),
+        }
+    }
+
+    /// Returns true if the value has been initialized.
+    #[inline(always)]
+    pub fn initialized(&self) -> bool {
+        let initialized = self.initialized.load(Ordering::Acquire);
+        if initialized {
+            return true;
+        }
+        return false;
+    }
+
+    /// Ensures that this lazy value is initialized. If the value has not
+    /// yet been initialized, this will raise a panic.
+    #[inline(always)]
+    pub fn ensure(&self) {
+        if self.initialized() {
+            return;
+        }
+        panic!("Lazy value was not initialized");
+    }
+
+    pub fn init(&self, value: T) {
+        assert!(!self.initialized());
+
+        // We need this lock to ensure that only one thread calls the init function at the same time.
+        let _init_guard = self.init_lock.lock();
+        // Check again, in case another thread initialized it while we were waiting for the lock.
+        let initialized = self.initialized();
+        if initialized {
+            return;
+        }
+        unsafe {
+            (*self.value.get()).as_mut_ptr().write(value);
+        }
+        self.initialized.store(true, Ordering::Release);
+    }
+    /// Forces the evaluation of this lazy value and returns a reference to
+    /// the result. This is equivalent to the `Deref` impl, but is explicit.
+    /// This will initialize the value if it has not yet been initialized.
+    pub fn get(&self) -> &T {
+        self.ensure();
+        return unsafe { self.get_unchecked() };
+    }
+
+    /// Returns a reference to the value if it has been initialized.
+    /// Otherwise, returns `None`.
+    pub fn try_get(&self) -> Option<&T> {
+        if self.initialized() {
+            return Some(unsafe { self.get_unchecked() });
+        }
+        return None;
+    }
+
+    /// Forces the evaluation of this lazy value and returns a mutable
+    /// reference to the result. This is equivalent to the `DerefMut` impl,
+    /// but is explicit. This will initialize the value if it has not yet
+    /// been initialized.
+    pub fn get_mut(&mut self) -> &mut T {
+        self.ensure();
+        return unsafe { self.get_mut_unchecked() };
+    }
+
+    #[inline(always)]
+    pub unsafe fn get_unchecked(&self) -> &T {
+        return &*(*self.value.get()).as_ptr();
+    }
+
+    #[inline(always)]
+    pub unsafe fn get_mut_unchecked(&mut self) -> &mut T {
+        return &mut *(*self.value.get()).as_mut_ptr();
+    }
+}
+
+impl<T: Sync> Deref for Lazy<T> {
+    type Target = T;
+
+    #[inline(always)]
+    fn deref(&self) -> &T {
+        return self.get();
+    }
+}
+
+impl<T: Sync> DerefMut for Lazy<T> {
+    #[inline(always)]
+    fn deref_mut(&mut self) -> &mut T {
+        return self.get_mut();
+    }
+}
+
+impl<T: Sync + Debug> Debug for Lazy<T> {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
+        if let Some(value) = self.try_get() {
+            return write!(f, "Lazy({:?})", value);
+        } else {
+            return write!(f, "Lazy(uninitialized)");
+        }
+    }
+}
+
+impl<T: Sync> Drop for Lazy<T> {
+    fn drop(&mut self) {
+        if self.initialized() {
+            unsafe {
+                (*self.value.get()).as_mut_ptr().drop_in_place();
+            }
+        }
+    }
+}

kernel/src/libs/mod.rs

@@ -13,5 +13,6 @@ pub mod spinlock;
 pub mod vec_cursor;
 #[macro_use]
 pub mod volatile;
-pub mod wait_queue;
 pub mod keyboard_parser;
+pub mod lazy_init;
+pub mod wait_queue;