mod.rs

//! Synchronization primitives.
//!
//! This module is an async version of [`std::sync`].
//!
//! [`std::sync`]: https://doc.rust-lang.org/std/sync/index.html
//!
//! ## The need for synchronization
//!
//! async-std's sync primitives are scheduler-aware, making it possible to
//! `.await` their operations - for example the locking of a [`Mutex`].
//!
//! Conceptually, a Rust program is a series of operations which will
//! be executed on a computer. The timeline of events happening in the
//! program is consistent with the order of the operations in the code.
//!
//! Consider the following code, operating on some global static variables:
//!
//! ```
//! static mut A: u32 = 0;
//! static mut B: u32 = 0;
//! static mut C: u32 = 0;
//!
//! fn main() {
//!     unsafe {
//!         A = 3;
//!         B = 4;
//!         A = A + B;
//!         C = B;
//!         println!("{} {} {}", A, B, C);
//!         C = A;
//!     }
//! }
//! ```
//!
//! It appears as if some variables stored in memory are changed, an addition
//! is performed, result is stored in `A` and the variable `C` is
//! modified twice.
//!
//! When only a single thread is involved, the results are as expected:
//! the line `7 4 4` gets printed.
//!
//! As for what happens behind the scenes, when optimizations are enabled the
//! final generated machine code might look very different from the code:
//!
//! - The first store to `C` might be moved before the store to `A` or `B`,
//!   _as if_ we had written `C = 4; A = 3; B = 4`.
//!
//! - Assignment of `A + B` to `A` might be removed, since the sum can be stored
//!   in a temporary location until it gets printed, with the global variable
//!   never getting updated.
//!
//! - The final result could be determined just by looking at the code
//!   at compile time, so [constant folding] might turn the whole
//!   block into a simple `println!("7 4 4")`.
//!
//! The compiler is allowed to perform any combination of these
//! optimizations, as long as the final optimized code, when executed,
//! produces the same results as the one without optimizations.
//!
//! Due to the [concurrency] involved in modern computers, assumptions
//! about the program's execution order are often wrong. Access to
//! global variables can lead to nondeterministic results, **even if**
//! compiler optimizations are disabled, and it is **still possible**
//! to introduce synchronization bugs.
//!
//! Note that thanks to Rust's safety guarantees, accessing global (static)
//! variables requires `unsafe` code, assuming we don't use any of the
//! synchronization primitives in this module.
//!
//! [constant folding]: https://en.wikipedia.org/wiki/Constant_folding
//! [concurrency]: https://en.wikipedia.org/wiki/Concurrency_(computer_science)
//!
//! ## Out-of-order execution
//!
//! Instructions can execute in a different order from the one we define, due to
//! various reasons:
//!
//! - The **compiler** reordering instructions: If the compiler can issue an
//!   instruction at an earlier point, it will try to do so. For example, it
//!   might hoist memory loads at the top of a code block, so that the CPU can
//!   start [prefetching] the values from memory.
//!
//!   In single-threaded scenarios, this can cause issues when writing
//!   signal handlers or certain kinds of low-level code.
//!   Use [compiler fences] to prevent this reordering.
//!
//! - A **single processor** executing instructions [out-of-order]:
//!   Modern CPUs are capable of [superscalar] execution,
//!   i.e., multiple instructions might be executing at the same time,
//!   even though the machine code describes a sequential process.
//!
//!   This kind of reordering is handled transparently by the CPU.
//!
//! - A **multiprocessor** system executing multiple hardware threads
//!   at the same time: In multi-threaded scenarios, you can use two
//!   kinds of primitives to deal with synchronization:
//!   - [memory fences] to ensure memory accesses are made visible to
//!   other CPUs in the right order.
//!   - [atomic operations] to ensure simultaneous access to the same
//!   memory location doesn't lead to undefined behavior.
//!
//! [prefetching]: https://en.wikipedia.org/wiki/Cache_prefetching
//! [compiler fences]: https://doc.rust-lang.org/std/sync/atomic/fn.compiler_fence.html
//! [out-of-order]: https://en.wikipedia.org/wiki/Out-of-order_execution
//! [superscalar]: https://en.wikipedia.org/wiki/Superscalar_processor
//! [memory fences]: https://doc.rust-lang.org/std/sync/atomic/fn.fence.html
//! [atomic operations]: https://doc.rust-lang.org/std/sync/atomic/index.html
//!
//! ## Higher-level synchronization objects
//!
//! Most of the low-level synchronization primitives are quite error-prone and
//! inconvenient to use, which is why async-std also exposes some
//! higher-level synchronization objects.
//!
//! These abstractions can be built out of lower-level primitives.
//! For efficiency, the sync objects in async-std are usually
//! implemented with help from the scheduler, which is
//! able to reschedule the tasks while they are blocked on acquiring
//! a lock.
//!
//! The following is an overview of the available synchronization
//! objects:
//!
//! - [`Arc`]: Atomically Reference-Counted pointer, which can be used
//!   in multithreaded environments to prolong the lifetime of some
//!   data until all the threads have finished using it.
//!
//! - [`Barrier`]: Ensures multiple threads will wait for each other
//!   to reach a point in the program, before continuing execution all
//!   together.
//!
//! - [`channel`]: Multi-producer, multi-consumer queues, used for
//!   message-based communication. Can provide a lightweight
//!   inter-task synchronisation mechanism, at the cost of some
//!   extra memory.
//!
//! - [`Mutex`]: Mutual exclusion mechanism, which ensures that at
//!   most one task at a time is able to access some data.
//!
//! - [`RwLock`]: Provides a mutual exclusion mechanism which allows
//!   multiple readers at the same time, while allowing only one
//!   writer at a time. In some cases, this can be more efficient than
//!   a mutex.
//!
//! [`Arc`]: struct.Arc.html
//! [`Barrier`]: struct.Barrier.html
//! [`channel`]: fn.channel.html
//! [`Mutex`]: struct.Mutex.html
//! [`RwLock`]: struct.RwLock.html
//!
//! # Examples
//!
//! Spawn a task that updates an integer protected by a mutex:
//!
//! ```
//! # async_std::task::block_on(async {
//! #
//! use std::sync::Arc;
//!
//! use async_std::sync::Mutex;
//! use async_std::task;
//!
//! let m1 = Arc::new(Mutex::new(0));
//! let m2 = m1.clone();
//!
//! task::spawn(async move {
//!     *m2.lock().await = 1;
//! })
//! .await;
//!
//! assert_eq!(*m1.lock().await, 1);
//! #
//! # })
//! ```

#![allow(clippy::needless_doctest_main)]

#[doc(inline)]
pub use std::sync::{Arc, Weak};

pub use mutex::{Mutex, MutexGuard};
pub use rwlock::{RwLock, RwLockReadGuard, RwLockWriteGuard};

mod mutex;
mod rwlock;

cfg_default! {
    pub(crate) mod atomic;
}

cfg_unstable! {
    pub use barrier::{Barrier, BarrierWaitResult};
    pub use channel::{channel, Sender, Receiver};

    mod barrier;
    mod channel;
}

pub(crate) mod waker_set;
pub(crate) use waker_set::WakerSet;