vendor/crossbeam-epoch-0.8.2/src/lib.rs

   1 //! Epoch-based memory reclamation.
   2 //!
   3 //! An interesting problem concurrent collections deal with comes from the remove operation.
   4 //! Suppose that a thread removes an element from a lock-free map, while another thread is reading
   5 //! that same element at the same time. The first thread must wait until the second thread stops
   6 //! reading the element. Only then it is safe to destruct it.
   7 //!
   8 //! Programming languages that come with garbage collectors solve this problem trivially. The
   9 //! garbage collector will destruct the removed element when no thread can hold a reference to it
  10 //! anymore.
  11 //!
  12 //! This crate implements a basic memory reclamation mechanism, which is based on epochs. When an
  13 //! element gets removed from a concurrent collection, it is inserted into a pile of garbage and
  14 //! marked with the current epoch. Every time a thread accesses a collection, it checks the current
  15 //! epoch, attempts to increment it, and destructs some garbage that became so old that no thread
  16 //! can be referencing it anymore.
  17 //!
  18 //! That is the general mechanism behind epoch-based memory reclamation, but the details are a bit
  19 //! more complicated. Anyhow, memory reclamation is designed to be fully automatic and something
  20 //! users of concurrent collections don't have to worry much about.
  21 //!
  22 //! # Pointers
  23 //!
  24 //! Concurrent collections are built using atomic pointers. This module provides [`Atomic`], which
  25 //! is just a shared atomic pointer to a heap-allocated object. Loading an [`Atomic`] yields a
  26 //! [`Shared`], which is an epoch-protected pointer through which the loaded object can be safely
  27 //! read.
  28 //!
  29 //! # Pinning
  30 //!
  31 //! Before an [`Atomic`] can be loaded, a participant must be [`pin`]ned. By pinning a participant
  32 //! we declare that any object that gets removed from now on must not be destructed just
  33 //! yet. Garbage collection of newly removed objects is suspended until the participant gets
  34 //! unpinned.
  35 //!
  36 //! # Garbage
  37 //!
  38 //! Objects that get removed from concurrent collections must be stashed away until all currently
  39 //! pinned participants get unpinned. Such objects can be stored into a thread-local or global
  40 //! storage, where they are kept until the right time for their destruction comes.
  41 //!
  42 //! There is a global shared instance of garbage queue. You can [`defer`] the execution of an
  43 //! arbitrary function until the global epoch is advanced enough. Most notably, concurrent data
  44 //! structures may defer the deallocation of an object.
  45 //!
  46 //! # APIs
  47 //!
  48 //! For majority of use cases, just use the default garbage collector by invoking [`pin`]. If you
  49 //! want to create your own garbage collector, use the [`Collector`] API.
  50 //!
  51 //! [`Atomic`]: struct.Atomic.html
  52 //! [`Collector`]: struct.Collector.html
  53 //! [`Shared`]: struct.Shared.html
  54 //! [`pin`]: fn.pin.html
  55 //! [`defer`]: struct.Guard.html#method.defer
  56
  57 #![warn(missing_docs)]
  58 #![warn(missing_debug_implementations)]
  59 #![cfg_attr(not(feature = "std"), no_std)]
  60 #![cfg_attr(feature = "nightly", feature(cfg_target_has_atomic))]
  61
  62 #[macro_use]
  63 extern crate cfg_if;
  64 #[cfg(feature = "std")]
  65 extern crate core;
  66
  67 extern crate maybe_uninit;
  68
  69 cfg_if! {
  70     if #[cfg(feature = "alloc")] {
  71         extern crate alloc;
  72     } else if #[cfg(feature = "std")] {
  73         extern crate std as alloc;
  74     }
  75 }
  76
  77 #[cfg_attr(feature = "nightly", cfg(target_has_atomic = "ptr"))]
  78 cfg_if! {
  79     if #[cfg(any(feature = "alloc", feature = "std"))] {
  80         extern crate crossbeam_utils;
  81         #[macro_use]
  82         extern crate memoffset;
  83         #[macro_use]
  84         extern crate scopeguard;
  85
  86         mod atomic;
  87         mod collector;
  88         mod deferred;
  89         mod epoch;
  90         mod guard;
  91         mod internal;
  92         mod sync;
  93
  94         pub use self::atomic::{Atomic, CompareAndSetError, CompareAndSetOrdering, Owned, Pointer, Shared};
  95         pub use self::collector::{Collector, LocalHandle};
  96         pub use self::guard::{unprotected, Guard};
  97     }
  98 }
  99
 100 cfg_if! {
 101     if #[cfg(feature = "std")] {
 102         #[macro_use]
 103         extern crate lazy_static;
 104
 105         mod default;
 106         pub use self::default::{default_collector, is_pinned, pin};
 107     }
 108 }