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|
use super::abi;
use crate::{
cell::UnsafeCell,
mem::MaybeUninit,
sync::atomic::{AtomicBool, AtomicUsize, Ordering},
};
/// A mutex implemented by `dis_dsp` (for intra-core synchronization) and a
/// spinlock (for inter-core synchronization).
pub struct SpinMutex<T = ()> {
locked: AtomicBool,
data: UnsafeCell<T>,
}
impl<T> SpinMutex<T> {
#[inline]
pub const fn new(x: T) -> Self {
Self { locked: AtomicBool::new(false), data: UnsafeCell::new(x) }
}
/// Acquire a lock.
#[inline]
pub fn with_locked<R>(&self, f: impl FnOnce(&mut T) -> R) -> R {
struct SpinMutexGuard<'a>(&'a AtomicBool);
impl Drop for SpinMutexGuard<'_> {
#[inline]
fn drop(&mut self) {
self.0.store(false, Ordering::Release);
unsafe { abi::ena_dsp() };
}
}
let _guard;
if unsafe { abi::sns_dsp() } == 0 {
let er = unsafe { abi::dis_dsp() };
debug_assert!(er >= 0);
// Wait until the current processor acquires a lock.
while self.locked.swap(true, Ordering::Acquire) {}
_guard = SpinMutexGuard(&self.locked);
}
f(unsafe { &mut *self.data.get() })
}
}
/// `OnceCell<(abi::ID, T)>` implemented by `dis_dsp` (for intra-core
/// synchronization) and a spinlock (for inter-core synchronization).
///
/// It's assumed that `0` is not a valid ID, and all kernel
/// object IDs fall into range `1..=usize::MAX`.
pub struct SpinIdOnceCell<T = ()> {
id: AtomicUsize,
spin: SpinMutex<()>,
extra: UnsafeCell<MaybeUninit<T>>,
}
const ID_UNINIT: usize = 0;
impl<T> SpinIdOnceCell<T> {
#[inline]
pub const fn new() -> Self {
Self {
id: AtomicUsize::new(ID_UNINIT),
extra: UnsafeCell::new(MaybeUninit::uninit()),
spin: SpinMutex::new(()),
}
}
#[inline]
pub fn get(&self) -> Option<(abi::ID, &T)> {
match self.id.load(Ordering::Acquire) {
ID_UNINIT => None,
id => Some((id as abi::ID, unsafe { (&*self.extra.get()).assume_init_ref() })),
}
}
#[inline]
pub fn get_mut(&mut self) -> Option<(abi::ID, &mut T)> {
match *self.id.get_mut() {
ID_UNINIT => None,
id => Some((id as abi::ID, unsafe { (&mut *self.extra.get()).assume_init_mut() })),
}
}
#[inline]
pub unsafe fn get_unchecked(&self) -> (abi::ID, &T) {
(self.id.load(Ordering::Acquire) as abi::ID, unsafe {
(&*self.extra.get()).assume_init_ref()
})
}
/// Assign the content without checking if it's already initialized or
/// being initialized.
pub unsafe fn set_unchecked(&self, (id, extra): (abi::ID, T)) {
debug_assert!(self.get().is_none());
// Assumption: A positive `abi::ID` fits in `usize`.
debug_assert!(id >= 0);
debug_assert!(usize::try_from(id).is_ok());
let id = id as usize;
unsafe { *self.extra.get() = MaybeUninit::new(extra) };
self.id.store(id, Ordering::Release);
}
/// Gets the contents of the cell, initializing it with `f` if
/// the cell was empty. If the cell was empty and `f` failed, an
/// error is returned.
///
/// Warning: `f` must not perform a blocking operation, which
/// includes panicking.
#[inline]
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<(abi::ID, &T), E>
where
F: FnOnce() -> Result<(abi::ID, T), E>,
{
// Fast path
if let Some(x) = self.get() {
return Ok(x);
}
self.initialize(f)?;
debug_assert!(self.get().is_some());
// Safety: The inner value has been initialized
Ok(unsafe { self.get_unchecked() })
}
fn initialize<F, E>(&self, f: F) -> Result<(), E>
where
F: FnOnce() -> Result<(abi::ID, T), E>,
{
self.spin.with_locked(|_| {
if self.id.load(Ordering::Relaxed) == ID_UNINIT {
let (initialized_id, initialized_extra) = f()?;
// Assumption: A positive `abi::ID` fits in `usize`.
debug_assert!(initialized_id >= 0);
debug_assert!(usize::try_from(initialized_id).is_ok());
let initialized_id = initialized_id as usize;
// Store the initialized contents. Use the release ordering to
// make sure the write is visible to the callers of `get`.
unsafe { *self.extra.get() = MaybeUninit::new(initialized_extra) };
self.id.store(initialized_id, Ordering::Release);
}
Ok(())
})
}
}
impl<T> Drop for SpinIdOnceCell<T> {
#[inline]
fn drop(&mut self) {
if self.get_mut().is_some() {
unsafe { (&mut *self.extra.get()).assume_init_drop() };
}
}
}
|
