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::Ordering;
11
12// Ideally, we want to always use AtomicU64, but since it is not available on all platforms,
13// we only use it when it is available for now.
14// TODO: On platforms where AtomicU64 is unavailable, we may want to use AtomicCell instead of AtomicUsize.
15#[cfg(target_has_atomic = "64")]
16type AtomicEpochRepr = crate::primitive::sync::atomic::AtomicU64;
17#[cfg(not(target_has_atomic = "64"))]
18type AtomicEpochRepr = crate::primitive::sync::atomic::AtomicUsize;
19#[cfg(target_has_atomic = "64")]
20type EpochRepr = u64;
21#[cfg(not(target_has_atomic = "64"))]
22type EpochRepr = usize;
23#[cfg(target_has_atomic = "64")]
24type EpochReprSigned = i64;
25#[cfg(not(target_has_atomic = "64"))]
26type EpochReprSigned = isize;
27
28/// An epoch that can be marked as pinned or unpinned.
29///
30/// Internally, the epoch is represented as an integer that wraps around at some unspecified point
31/// and a flag that represents whether it is pinned or unpinned.
32#[derive(Copy, Clone, Default, Debug, Eq, PartialEq)]
33pub(crate) struct Epoch {
34    /// The least significant bit is set if pinned. The rest of the bits hold the epoch.
35    data: EpochRepr,
36}
37
38impl Epoch {
39    /// Returns the starting epoch in unpinned state.
40    #[inline]
41    pub(crate) fn starting() -> Self {
42        Self::default()
43    }
44
45    /// Returns the number of epochs `self` is ahead of `rhs`.
46    ///
47    /// Internally, epochs are represented as numbers in the range `(isize::MIN / 2) .. (isize::MAX
48    /// / 2)`, so the returned distance will be in the same interval.
49    pub(crate) fn wrapping_sub(self, rhs: Self) -> EpochReprSigned {
50        // The result is the same with `(self.data & !1).wrapping_sub(rhs.data & !1) as isize >> 1`,
51        // because the possible difference of LSB in `(self.data & !1).wrapping_sub(rhs.data & !1)`
52        // will be ignored in the shift operation.
53        self.data.wrapping_sub(rhs.data & !1) as EpochReprSigned >> 1
54    }
55
56    /// Returns `true` if the epoch is marked as pinned.
57    #[inline]
58    pub(crate) fn is_pinned(self) -> bool {
59        (self.data & 1) == 1
60    }
61
62    /// Returns the same epoch, but marked as pinned.
63    #[inline]
64    pub(crate) fn pinned(self) -> Epoch {
65        Epoch {
66            data: self.data | 1,
67        }
68    }
69
70    /// Returns the same epoch, but marked as unpinned.
71    #[inline]
72    pub(crate) fn unpinned(self) -> Epoch {
73        Epoch {
74            data: self.data & !1,
75        }
76    }
77
78    /// Returns the successor epoch.
79    ///
80    /// The returned epoch will be marked as pinned only if the previous one was as well.
81    #[inline]
82    pub(crate) fn successor(self) -> Epoch {
83        Epoch {
84            data: self.data.wrapping_add(2),
85        }
86    }
87}
88
89/// An atomic value that holds an `Epoch`.
90#[derive(Default, Debug)]
91pub(crate) struct AtomicEpoch {
92    /// Since `Epoch` is just a wrapper around `usize`, an `AtomicEpoch` is similarly represented
93    /// using an `AtomicUsize`.
94    data: AtomicEpochRepr,
95}
96
97impl AtomicEpoch {
98    /// Creates a new atomic epoch.
99    #[inline]
100    pub(crate) fn new(epoch: Epoch) -> Self {
101        let data = AtomicEpochRepr::new(epoch.data);
102        Self { data }
103    }
104
105    /// Loads a value from the atomic epoch.
106    #[inline]
107    pub(crate) fn load(&self, ord: Ordering) -> Epoch {
108        Epoch {
109            data: self.data.load(ord),
110        }
111    }
112
113    /// Stores a value into the atomic epoch.
114    #[inline]
115    pub(crate) fn store(&self, epoch: Epoch, ord: Ordering) {
116        self.data.store(epoch.data, ord);
117    }
118
119    /// Stores a value into the atomic epoch if the current value is the same as `current`.
120    ///
121    /// The return value is a result indicating whether the new value was written and containing
122    /// the previous value. On success this value is guaranteed to be equal to `current`.
123    ///
124    /// This method takes two `Ordering` arguments to describe the memory
125    /// ordering of this operation. `success` describes the required ordering for the
126    /// read-modify-write operation that takes place if the comparison with `current` succeeds.
127    /// `failure` describes the required ordering for the load operation that takes place when
128    /// the comparison fails. Using `Acquire` as success ordering makes the store part
129    /// of this operation `Relaxed`, and using `Release` makes the successful load
130    /// `Relaxed`. The failure ordering can only be `SeqCst`, `Acquire` or `Relaxed`
131    /// and must be equivalent to or weaker than the success ordering.
132    #[inline]
133    pub(crate) fn compare_exchange(
134        &self,
135        current: Epoch,
136        new: Epoch,
137        success: Ordering,
138        failure: Ordering,
139    ) -> Result<Epoch, Epoch> {
140        match self
141            .data
142            .compare_exchange(current.data, new.data, success, failure)
143        {
144            Ok(data) => Ok(Epoch { data }),
145            Err(data) => Err(Epoch { data }),
146        }
147    }
148}
crossbeam_epoch/epoch.rs

crossbeam_epoch/
epoch.rs
