crossbeam_epoch/
epoch.rs

1//! The global epoch
2//!
3//! The last bit in this number is unused and is always zero. Every so often the global epoch is
4//! incremented, i.e. we say it "advances". A pinned participant may advance the global epoch only
5//! if all currently pinned participants have been pinned in the current epoch.
6//!
7//! If an object became garbage in some epoch, then we can be sure that after two advancements no
8//! participant will hold a reference to it. That is the crux of safe memory reclamation.
9
10use crate::primitive::sync::atomic::{AtomicUsize, Ordering};
11
12/// An epoch that can be marked as pinned or unpinned.
13///
14/// Internally, the epoch is represented as an integer that wraps around at some unspecified point
15/// and a flag that represents whether it is pinned or unpinned.
16#[derive(Copy, Clone, Default, Debug, Eq, PartialEq)]
17pub(crate) struct Epoch {
18    /// The least significant bit is set if pinned. The rest of the bits hold the epoch.
19    data: usize,
20}
21
22impl Epoch {
23    /// Returns the starting epoch in unpinned state.
24    #[inline]
25    pub(crate) fn starting() -> Self {
26        Self::default()
27    }
28
29    /// Returns the number of epochs `self` is ahead of `rhs`.
30    ///
31    /// Internally, epochs are represented as numbers in the range `(isize::MIN / 2) .. (isize::MAX
32    /// / 2)`, so the returned distance will be in the same interval.
33    pub(crate) fn wrapping_sub(self, rhs: Self) -> isize {
34        // The result is the same with `(self.data & !1).wrapping_sub(rhs.data & !1) as isize >> 1`,
35        // because the possible difference of LSB in `(self.data & !1).wrapping_sub(rhs.data & !1)`
36        // will be ignored in the shift operation.
37        self.data.wrapping_sub(rhs.data & !1) as isize >> 1
38    }
39
40    /// Returns `true` if the epoch is marked as pinned.
41    #[inline]
42    pub(crate) fn is_pinned(self) -> bool {
43        (self.data & 1) == 1
44    }
45
46    /// Returns the same epoch, but marked as pinned.
47    #[inline]
48    pub(crate) fn pinned(self) -> Epoch {
49        Epoch {
50            data: self.data | 1,
51        }
52    }
53
54    /// Returns the same epoch, but marked as unpinned.
55    #[inline]
56    pub(crate) fn unpinned(self) -> Epoch {
57        Epoch {
58            data: self.data & !1,
59        }
60    }
61
62    /// Returns the successor epoch.
63    ///
64    /// The returned epoch will be marked as pinned only if the previous one was as well.
65    #[inline]
66    pub(crate) fn successor(self) -> Epoch {
67        Epoch {
68            data: self.data.wrapping_add(2),
69        }
70    }
71}
72
73/// An atomic value that holds an `Epoch`.
74#[derive(Default, Debug)]
75pub(crate) struct AtomicEpoch {
76    /// Since `Epoch` is just a wrapper around `usize`, an `AtomicEpoch` is similarly represented
77    /// using an `AtomicUsize`.
78    data: AtomicUsize,
79}
80
81impl AtomicEpoch {
82    /// Creates a new atomic epoch.
83    #[inline]
84    pub(crate) fn new(epoch: Epoch) -> Self {
85        let data = AtomicUsize::new(epoch.data);
86        AtomicEpoch { data }
87    }
88
89    /// Loads a value from the atomic epoch.
90    #[inline]
91    pub(crate) fn load(&self, ord: Ordering) -> Epoch {
92        Epoch {
93            data: self.data.load(ord),
94        }
95    }
96
97    /// Stores a value into the atomic epoch.
98    #[inline]
99    pub(crate) fn store(&self, epoch: Epoch, ord: Ordering) {
100        self.data.store(epoch.data, ord);
101    }
102
103    /// Stores a value into the atomic epoch if the current value is the same as `current`.
104    ///
105    /// The return value is a result indicating whether the new value was written and containing
106    /// the previous value. On success this value is guaranteed to be equal to `current`.
107    ///
108    /// This method takes two `Ordering` arguments to describe the memory
109    /// ordering of this operation. `success` describes the required ordering for the
110    /// read-modify-write operation that takes place if the comparison with `current` succeeds.
111    /// `failure` describes the required ordering for the load operation that takes place when
112    /// the comparison fails. Using `Acquire` as success ordering makes the store part
113    /// of this operation `Relaxed`, and using `Release` makes the successful load
114    /// `Relaxed`. The failure ordering can only be `SeqCst`, `Acquire` or `Relaxed`
115    /// and must be equivalent to or weaker than the success ordering.
116    #[inline]
117    pub(crate) fn compare_exchange(
118        &self,
119        current: Epoch,
120        new: Epoch,
121        success: Ordering,
122        failure: Ordering,
123    ) -> Result<Epoch, Epoch> {
124        match self
125            .data
126            .compare_exchange(current.data, new.data, success, failure)
127        {
128            Ok(data) => Ok(Epoch { data }),
129            Err(data) => Err(Epoch { data }),
130        }
131    }
132}
crossbeam_epoch/epoch.rs

crossbeam_epoch/
epoch.rs
