tokio/util/
sharded_list.rs

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1use std::ptr::NonNull;
2use std::sync::atomic::Ordering;
3
4use crate::loom::sync::{Mutex, MutexGuard};
5use crate::util::metric_atomics::{MetricAtomicU64, MetricAtomicUsize};
6
7use super::linked_list::{Link, LinkedList};
8
9/// An intrusive linked list supporting highly concurrent updates.
10///
11/// It currently relies on `LinkedList`, so it is the caller's
12/// responsibility to ensure the list is empty before dropping it.
13///
14/// Note: Due to its inner sharded design, the order of nodes cannot be guaranteed.
15pub(crate) struct ShardedList<L, T> {
16    lists: Box<[Mutex<LinkedList<L, T>>]>,
17    added: MetricAtomicU64,
18    count: MetricAtomicUsize,
19    shard_mask: usize,
20}
21
22/// Determines which linked list an item should be stored in.
23///
24/// # Safety
25///
26/// Implementations must guarantee that the id of an item does not change from
27/// call to call.
28pub(crate) unsafe trait ShardedListItem: Link {
29    /// # Safety
30    /// The provided pointer must point at a valid list item.
31    unsafe fn get_shard_id(target: NonNull<Self::Target>) -> usize;
32}
33
34impl<L, T> ShardedList<L, T> {
35    /// Creates a new and empty sharded linked list with the specified size.
36    pub(crate) fn new(sharded_size: usize) -> Self {
37        assert!(sharded_size.is_power_of_two());
38
39        let shard_mask = sharded_size - 1;
40        let mut lists = Vec::with_capacity(sharded_size);
41        for _ in 0..sharded_size {
42            lists.push(Mutex::new(LinkedList::<L, T>::new()))
43        }
44        Self {
45            lists: lists.into_boxed_slice(),
46            added: MetricAtomicU64::new(0),
47            count: MetricAtomicUsize::new(0),
48            shard_mask,
49        }
50    }
51}
52
53/// Used to get the lock of shard.
54pub(crate) struct ShardGuard<'a, L, T> {
55    lock: MutexGuard<'a, LinkedList<L, T>>,
56    added: &'a MetricAtomicU64,
57    count: &'a MetricAtomicUsize,
58    id: usize,
59}
60
61impl<L: ShardedListItem> ShardedList<L, L::Target> {
62    /// Removes the last element from a list specified by `shard_id` and returns it, or None if it is
63    /// empty.
64    pub(crate) fn pop_back(&self, shard_id: usize) -> Option<L::Handle> {
65        let mut lock = self.shard_inner(shard_id);
66        let node = lock.pop_back();
67        if node.is_some() {
68            self.count.decrement();
69        }
70        node
71    }
72
73    /// Removes the specified node from the list.
74    ///
75    /// # Safety
76    ///
77    /// The caller **must** ensure that exactly one of the following is true:
78    /// - `node` is currently contained by `self`,
79    /// - `node` is not contained by any list,
80    /// - `node` is currently contained by some other `GuardedLinkedList`.
81    pub(crate) unsafe fn remove(&self, node: NonNull<L::Target>) -> Option<L::Handle> {
82        let id = L::get_shard_id(node);
83        let mut lock = self.shard_inner(id);
84        // SAFETY: Since the shard id cannot change, it's not possible for this node
85        // to be in any other list of the same sharded list.
86        let node = unsafe { lock.remove(node) };
87        if node.is_some() {
88            self.count.decrement();
89        }
90        node
91    }
92
93    /// Gets the lock of `ShardedList`, makes us have the write permission.
94    pub(crate) fn lock_shard(&self, val: &L::Handle) -> ShardGuard<'_, L, L::Target> {
95        let id = unsafe { L::get_shard_id(L::as_raw(val)) };
96        ShardGuard {
97            lock: self.shard_inner(id),
98            added: &self.added,
99            count: &self.count,
100            id,
101        }
102    }
103
104    /// Gets the count of elements in this list.
105    pub(crate) fn len(&self) -> usize {
106        self.count.load(Ordering::Relaxed)
107    }
108
109    cfg_64bit_metrics! {
110        /// Gets the total number of elements added to this list.
111        pub(crate) fn added(&self) -> u64 {
112            self.added.load(Ordering::Relaxed)
113        }
114    }
115
116    /// Returns whether the linked list does not contain any node.
117    pub(crate) fn is_empty(&self) -> bool {
118        self.len() == 0
119    }
120
121    /// Gets the shard size of this `SharedList`.
122    ///
123    /// Used to help us to decide the parameter `shard_id` of the `pop_back` method.
124    pub(crate) fn shard_size(&self) -> usize {
125        self.shard_mask + 1
126    }
127
128    #[inline]
129    fn shard_inner(&self, id: usize) -> MutexGuard<'_, LinkedList<L, <L as Link>::Target>> {
130        // Safety: This modulo operation ensures that the index is not out of bounds.
131        unsafe { self.lists.get_unchecked(id & self.shard_mask).lock() }
132    }
133}
134
135impl<'a, L: ShardedListItem> ShardGuard<'a, L, L::Target> {
136    /// Push a value to this shard.
137    pub(crate) fn push(mut self, val: L::Handle) {
138        let id = unsafe { L::get_shard_id(L::as_raw(&val)) };
139        assert_eq!(id, self.id);
140        self.lock.push_front(val);
141        self.added.add(1, Ordering::Relaxed);
142        self.count.increment();
143    }
144}
145
146cfg_taskdump! {
147    impl<L: ShardedListItem> ShardedList<L, L::Target> {
148        pub(crate) fn for_each<F>(&self, mut f: F)
149        where
150            F: FnMut(&L::Handle),
151        {
152            let mut guards = Vec::with_capacity(self.lists.len());
153            for list in self.lists.iter() {
154                guards.push(list.lock());
155            }
156            for g in &mut guards {
157                g.for_each(&mut f);
158            }
159        }
160    }
161}
tokio/util/sharded_list.rs

tokio/util/
sharded_list.rs
