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use std::hint;
use std::sync::atomic::{AtomicPtr, Ordering};
use uniset::{AtomicBitSet, BitSet};
use crate::lock::{LockExclusiveGuard, LockSharedGuard, RwLock};
/// A wake set which allows us to immutably set an index.
pub(crate) struct WakeSet {
set: AtomicBitSet,
/// Read locks are held every time someone manipulates the underlying set,
/// we then (briefly) acquire a write lock to get unique access, after we
/// have swapped out the wake set pointer.
///
/// We keep this lock separate since we operate over raw pointers, and we'd
/// like the ability to "shed" the lock from `LocalWakeSet` when
/// appropriate. I.e. we don't want to keep track of a lock guard, but we
/// still have a region of operation where we want to consider the wake set
/// as exclusively owned.
lock: RwLock,
}
impl WakeSet {
pub(crate) fn new() -> Self {
Self {
set: AtomicBitSet::new(),
lock: RwLock::new(),
}
}
pub(crate) fn locked() -> Self {
Self {
set: AtomicBitSet::new(),
lock: RwLock::locked(),
}
}
pub(crate) fn new_raw() -> *mut Self {
Box::into_raw(Box::new(Self::new()))
}
/// Try to lock the current thread until we have unique access.
pub(crate) fn try_lock_exclusive(&self) -> bool {
self.lock.try_lock_exclusive_immediate()
}
pub(crate) fn lock_exclusive(&self) {
while !self.try_lock_exclusive() {
hint::spin_loop();
}
}
pub(crate) unsafe fn unlock_exclusive(&self) {
self.lock.unlock_exclusive_immediate();
}
/// Lock interest in reading.
pub(crate) fn try_lock_shared(&self) -> Option<LockSharedGuard<'_>> {
self.lock.try_lock_shared()
}
/// Set the given index in the referenced bitset.
pub(crate) fn set(&self, index: usize) {
self.set.set(index);
}
/// Treat the bitset as a local, mutable BitSet.
///
/// Caller must ensure that they have unique access to the atomic bit set by
/// only using this while an exclusive lock is held through
/// `lock_exclusive`.
pub(crate) fn as_mut_set(&mut self) -> &mut BitSet {
self.set.as_local_mut()
}
}
pub(crate) struct SharedWakeSet {
wake_set: AtomicPtr<WakeSet>,
prevent_drop_lock: RwLock,
}
impl SharedWakeSet {
/// Construct a new shared wake set.
pub(crate) fn new() -> Self {
Self {
wake_set: AtomicPtr::new(WakeSet::new_raw()),
prevent_drop_lock: RwLock::new(),
}
}
/// Swap the current pointer with another.
pub(crate) fn swap(&self, other: *mut WakeSet) -> *mut WakeSet {
self.wake_set.swap(other, Ordering::AcqRel)
}
/// Register wakeup for the specified index.
pub(crate) fn wake(&self, index: usize) {
// We need to spin here, since the wake set might be swapped out while we
// are trying to update it.
while !self.try_wake(index) {}
}
/// Prevent that the pointer is being written to while this guard is being
/// held. This makes sure there are no readers in the critical section that
/// might read an invalid wake set while it's being deallocated.
pub(crate) fn prevent_drop_write(&self) -> LockExclusiveGuard<'_> {
loop {
if let Some(guard) = self.prevent_drop_lock.try_lock_exclusive_guard() {
return guard;
}
hint::spin_loop();
}
}
fn try_prevent_drop_read(&self) -> Option<LockSharedGuard<'_>> {
self.prevent_drop_lock.try_lock_shared()
}
fn try_wake(&self, index: usize) -> bool {
// Here is a critical section that becomes relevant if we are trying to
// drop the unordered set.
//
// This prevents the unordered set from being dropped while we hold this
// guard, which means that the pointer we read must always point to
// allocated memory. Otherwise a reader might get past the point that we
// loaded the pointer from `self`, but before we tried to acquire the
// read guard and set the index _while_ the active wake set is being
// dropped.
let _guard = match self.try_prevent_drop_read() {
Some(guard) => guard,
None => return false,
};
let wake_set = self.wake_set.load(Ordering::Acquire);
debug_assert!(!wake_set.is_null());
// Safety: We know wake_set references valid memory, because in order to
// have access to `SharedWakeSet`, we must also hold an `Arc` to it - either
// through a reference or by it being stored in `Internals`.
//
// There is however a short window in which the wake set has been swapped in
// `Unordered`, but at this point it is not possible for it to be
// invalidated. This can only happen if `Unordered` is dropped, and this
// does not happen while it's being polled.
let wake_set = unsafe { &*wake_set };
let _guard2 = match wake_set.try_lock_shared() {
Some(guard) => guard,
None => return false,
};
wake_set.set(index);
true
}
}
impl Drop for SharedWakeSet {
fn drop(&mut self) {
let wake_set = self.wake_set.load(Ordering::Acquire);
debug_assert!(!wake_set.is_null());
// Safety: At this point, there are no other ways to access the
// `SharedWakeSet`, so we are not racing against someone trying to call
// wake. Nor are we racing against `Unordered` dropping the wake set
// since this is the active set which has been swapped in exclusively.
unsafe {
drop(Box::from_raw(wake_set));
}
}
}