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
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187

//! Epoch-based memory reclamation.
//!
//! An interesting problem concurrent collections deal with comes from the remove operation.
//! Suppose that a thread removes an element from a lock-free map, while another thread is reading
//! that same element at the same time. The first thread must wait until the second thread stops
//! reading the element. Only then it is safe to destruct it.
//!
//! Programming languages that come with garbage collectors solve this problem trivially. The
//! garbage collector will destruct the removed element when no thread can hold a reference to it
//! anymore.
//!
//! This crate implements a basic memory reclamation mechanism, which is based on epochs. When an
//! element gets removed from a concurrent collection, it is inserted into a pile of garbage and
//! marked with the current epoch. Every time a thread accesses a collection, it checks the current
//! epoch, attempts to increment it, and destructs some garbage that became so old that no thread
//! can be referencing it anymore.
//!
//! That is the general mechanism behind epoch-based memory reclamation, but the details are a bit
//! more complicated. Anyhow, memory reclamation is designed to be fully automatic and something
//! users of concurrent collections don't have to worry much about.
//!
//! # Pointers
//!
//! Concurrent collections are built using atomic pointers. This module provides [`Atomic`], which
//! is just a shared atomic pointer to a heap-allocated object. Loading an [`Atomic`] yields a
//! [`Shared`], which is an epoch-protected pointer through which the loaded object can be safely
//! read.
//!
//! # Pinning
//!
//! Before an [`Atomic`] can be loaded, a participant must be [`pin`]ned. By pinning a participant
//! we declare that any object that gets removed from now on must not be destructed just
//! yet. Garbage collection of newly removed objects is suspended until the participant gets
//! unpinned.
//!
//! # Garbage
//!
//! Objects that get removed from concurrent collections must be stashed away until all currently
//! pinned participants get unpinned. Such objects can be stored into a thread-local or global
//! storage, where they are kept until the right time for their destruction comes.
//!
//! There is a global shared instance of garbage queue. You can [`defer`](Guard::defer) the execution of an
//! arbitrary function until the global epoch is advanced enough. Most notably, concurrent data
//! structures may defer the deallocation of an object.
//!
//! # APIs
//!
//! For majority of use cases, just use the default garbage collector by invoking [`pin`]. If you
//! want to create your own garbage collector, use the [`Collector`] API.

#![doc(test(
    no_crate_inject,
    attr(
        deny(warnings, rust_2018_idioms),
        allow(dead_code, unused_assignments, unused_variables)
    )
))]
#![warn(
    missing_docs,
    missing_debug_implementations,
    rust_2018_idioms,
    unreachable_pub
)]
#![cfg_attr(not(feature = "std"), no_std)]
#![cfg_attr(feature = "nightly", feature(cfg_target_has_atomic))]
#![cfg_attr(feature = "nightly", feature(const_fn))]

#[cfg(crossbeam_loom)]
extern crate loom_crate as loom;

use cfg_if::cfg_if;

#[cfg(crossbeam_loom)]
#[allow(unused_imports, dead_code)]
mod primitive {
    pub(crate) mod cell {
        pub(crate) use loom::cell::UnsafeCell;
    }
    pub(crate) mod sync {
        pub(crate) mod atomic {
            use core::sync::atomic::Ordering;
            pub(crate) use loom::sync::atomic::AtomicUsize;
            pub(crate) fn fence(ord: Ordering) {
                if let Ordering::Acquire = ord {
                } else {
                    // FIXME: loom only supports acquire fences at the moment.
                    // https://github.com/tokio-rs/loom/issues/117
                    // let's at least not panic...
                    // this may generate some false positives (`SeqCst` is stronger than `Acquire`
                    // for example), and some false negatives (`Relaxed` is weaker than `Acquire`),
                    // but it's the best we can do for the time being.
                }
                loom::sync::atomic::fence(Ordering::Acquire)
            }

            // FIXME: loom does not support compiler_fence at the moment.
            // https://github.com/tokio-rs/loom/issues/117
            // we use fence as a stand-in for compiler_fence for the time being.
            // this may miss some races since fence is stronger than compiler_fence,
            // but it's the best we can do for the time being.
            pub(crate) use self::fence as compiler_fence;
        }
        pub(crate) use loom::sync::Arc;
    }
    pub(crate) use loom::lazy_static;
    pub(crate) use loom::thread_local;
}
#[cfg(not(crossbeam_loom))]
#[allow(unused_imports, dead_code)]
mod primitive {
    #[cfg(any(feature = "alloc", feature = "std"))]
    pub(crate) mod cell {
        #[derive(Debug)]
        #[repr(transparent)]
        pub(crate) struct UnsafeCell<T>(::core::cell::UnsafeCell<T>);

        // loom's UnsafeCell has a slightly different API than the standard library UnsafeCell.
        // Since we want the rest of the code to be agnostic to whether it's running under loom or
        // not, we write this small wrapper that provides the loom-supported API for the standard
        // library UnsafeCell. This is also what the loom documentation recommends:
        // https://github.com/tokio-rs/loom#handling-loom-api-differences
        impl<T> UnsafeCell<T> {
            #[inline]
            pub(crate) fn new(data: T) -> UnsafeCell<T> {
                UnsafeCell(::core::cell::UnsafeCell::new(data))
            }

            #[inline]
            pub(crate) fn with<R>(&self, f: impl FnOnce(*const T) -> R) -> R {
                f(self.0.get())
            }

            #[inline]
            pub(crate) fn with_mut<R>(&self, f: impl FnOnce(*mut T) -> R) -> R {
                f(self.0.get())
            }
        }
    }
    #[cfg(any(feature = "alloc", feature = "std"))]
    pub(crate) mod sync {
        pub(crate) mod atomic {
            pub(crate) use core::sync::atomic::compiler_fence;
            pub(crate) use core::sync::atomic::fence;
            pub(crate) use core::sync::atomic::AtomicUsize;
        }
        #[cfg_attr(feature = "nightly", cfg(target_has_atomic = "ptr"))]
        pub(crate) use alloc::sync::Arc;
    }

    #[cfg(feature = "std")]
    pub(crate) use std::thread_local;

    #[cfg(feature = "std")]
    pub(crate) use lazy_static::lazy_static;
}

#[cfg_attr(feature = "nightly", cfg(target_has_atomic = "ptr"))]
cfg_if! {
    if #[cfg(feature = "alloc")] {
        extern crate alloc;

        mod atomic;
        mod collector;
        mod deferred;
        mod epoch;
        mod guard;
        mod internal;
        mod sync;

        pub use self::atomic::{
            Pointable, Atomic, CompareExchangeError,
            Owned, Pointer, Shared,
        };
        pub use self::collector::{Collector, LocalHandle};
        pub use self::guard::{unprotected, Guard};

        #[allow(deprecated)]
        pub use self::atomic::{CompareAndSetError, CompareAndSetOrdering};
    }
}

cfg_if! {
    if #[cfg(feature = "std")] {
        mod default;
        pub use self::default::{default_collector, is_pinned, pin};
    }
}