summaryrefslogtreecommitdiffstats
path: root/vendor/crossbeam-epoch/src/epoch.rs
blob: 663508bd7b00b37cb88f99caa1e74a685d27f8e2 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
//! The global epoch
//!
//! The last bit in this number is unused and is always zero. Every so often the global epoch is
//! incremented, i.e. we say it "advances". A pinned participant may advance the global epoch only
//! if all currently pinned participants have been pinned in the current epoch.
//!
//! If an object became garbage in some epoch, then we can be sure that after two advancements no
//! participant will hold a reference to it. That is the crux of safe memory reclamation.

use crate::primitive::sync::atomic::AtomicUsize;
use core::sync::atomic::Ordering;

/// An epoch that can be marked as pinned or unpinned.
///
/// Internally, the epoch is represented as an integer that wraps around at some unspecified point
/// and a flag that represents whether it is pinned or unpinned.
#[derive(Copy, Clone, Default, Debug, Eq, PartialEq)]
pub(crate) struct Epoch {
    /// The least significant bit is set if pinned. The rest of the bits hold the epoch.
    data: usize,
}

impl Epoch {
    /// Returns the starting epoch in unpinned state.
    #[inline]
    pub(crate) fn starting() -> Self {
        Self::default()
    }

    /// Returns the number of epochs `self` is ahead of `rhs`.
    ///
    /// Internally, epochs are represented as numbers in the range `(isize::MIN / 2) .. (isize::MAX
    /// / 2)`, so the returned distance will be in the same interval.
    pub(crate) fn wrapping_sub(self, rhs: Self) -> isize {
        // The result is the same with `(self.data & !1).wrapping_sub(rhs.data & !1) as isize >> 1`,
        // because the possible difference of LSB in `(self.data & !1).wrapping_sub(rhs.data & !1)`
        // will be ignored in the shift operation.
        self.data.wrapping_sub(rhs.data & !1) as isize >> 1
    }

    /// Returns `true` if the epoch is marked as pinned.
    #[inline]
    pub(crate) fn is_pinned(self) -> bool {
        (self.data & 1) == 1
    }

    /// Returns the same epoch, but marked as pinned.
    #[inline]
    pub(crate) fn pinned(self) -> Epoch {
        Epoch {
            data: self.data | 1,
        }
    }

    /// Returns the same epoch, but marked as unpinned.
    #[inline]
    pub(crate) fn unpinned(self) -> Epoch {
        Epoch {
            data: self.data & !1,
        }
    }

    /// Returns the successor epoch.
    ///
    /// The returned epoch will be marked as pinned only if the previous one was as well.
    #[inline]
    pub(crate) fn successor(self) -> Epoch {
        Epoch {
            data: self.data.wrapping_add(2),
        }
    }
}

/// An atomic value that holds an `Epoch`.
#[derive(Default, Debug)]
pub(crate) struct AtomicEpoch {
    /// Since `Epoch` is just a wrapper around `usize`, an `AtomicEpoch` is similarly represented
    /// using an `AtomicUsize`.
    data: AtomicUsize,
}

impl AtomicEpoch {
    /// Creates a new atomic epoch.
    #[inline]
    pub(crate) fn new(epoch: Epoch) -> Self {
        let data = AtomicUsize::new(epoch.data);
        AtomicEpoch { data }
    }

    /// Loads a value from the atomic epoch.
    #[inline]
    pub(crate) fn load(&self, ord: Ordering) -> Epoch {
        Epoch {
            data: self.data.load(ord),
        }
    }

    /// Stores a value into the atomic epoch.
    #[inline]
    pub(crate) fn store(&self, epoch: Epoch, ord: Ordering) {
        self.data.store(epoch.data, ord);
    }

    /// Stores a value into the atomic epoch if the current value is the same as `current`.
    ///
    /// The return value is a result indicating whether the new value was written and containing
    /// the previous value. On success this value is guaranteed to be equal to `current`.
    ///
    /// This method takes two `Ordering` arguments to describe the memory
    /// ordering of this operation. `success` describes the required ordering for the
    /// read-modify-write operation that takes place if the comparison with `current` succeeds.
    /// `failure` describes the required ordering for the load operation that takes place when
    /// the comparison fails. Using `Acquire` as success ordering makes the store part
    /// of this operation `Relaxed`, and using `Release` makes the successful load
    /// `Relaxed`. The failure ordering can only be `SeqCst`, `Acquire` or `Relaxed`
    /// and must be equivalent to or weaker than the success ordering.
    #[inline]
    pub(crate) fn compare_exchange(
        &self,
        current: Epoch,
        new: Epoch,
        success: Ordering,
        failure: Ordering,
    ) -> Result<Epoch, Epoch> {
        match self
            .data
            .compare_exchange(current.data, new.data, success, failure)
        {
            Ok(data) => Ok(Epoch { data }),
            Err(data) => Err(Epoch { data }),
        }
    }
}