Documentation (synchronization primitives)

Documentation (core parking lot API)

Documentation (type-safe lock API)

This library provides implementations of Mutex, RwLock, Condvar and Once that are smaller, faster and more flexible than those in the Rust standard library, as well as a ReentrantMutex type which supports recursive locking. It also exposes a low-level API for creating your own efficient synchronization primitives.

When tested on x86_64 Linux, parking_lot::Mutex was found to be 1.5x faster than std::sync::Mutex when uncontended, and up to 5x faster when contended from multiple threads. The numbers for RwLock vary depending on the number of reader and writer threads, but are almost always faster than the standard library RwLock, and even up to 50x faster in some cases.

The primitives provided by this library have several advantages over those in the Rust standard library:

1. Mutex and Once only require 1 byte of storage space, while Condvar and RwLock only require 1 word of storage space. On the other hand on some platforms (macOS and a few others) the standard library primitives require a dynamically allocated Box to hold OS-specific synchronization primitives. The small size of Mutex in particular encourages the use of fine-grained locks to increase parallelism.
2. Uncontended lock acquisition and release is done through fast inline paths which only require a single atomic operation.
3. Microcontention (a contended lock with a short critical section) is efficiently handled by spinning a few times while trying to acquire a lock.
4. The locks are adaptive and will suspend a thread after a few failed spin attempts. This makes the locks suitable for both long and short critical sections.
5. Condvar, RwLock and Once work on Windows XP, unlike the standard library versions of those types.
6. RwLock takes advantage of hardware lock elision on processors that support it, which can lead to huge performance wins with many readers. This must be enabled with the hardware-lock-elision feature.
7. RwLock uses a task-fair locking policy, which avoids reader and writer starvation, whereas the standard library version makes no guarantees.
8. Condvar is guaranteed not to produce spurious wakeups. A thread will only be woken up if it timed out or it was woken up by a notification.
9. Condvar::notify_all will only wake up a single thread and requeue the rest to wait on the associated Mutex. This avoids a thundering herd problem where all threads try to acquire the lock at the same time.
10. RwLock supports atomically downgrading a write lock into a read lock.
11. Mutex and RwLock allow raw unlocking without a RAII guard object.
12. Mutex<()> and RwLock<()> allow raw locking without a RAII guard object.
13. Mutex and RwLock support eventual fairness which allows them to be fair on average without sacrificing performance.
14. A ReentrantMutex type which supports recursive locking.
15. An experimental deadlock detector that works for Mutex, RwLock and ReentrantMutex. This feature is disabled by default and can be enabled via the deadlock_detection feature.
16. RwLock supports atomically upgrading an "upgradable" read lock into a write lock.
17. Optional support for serde. Enable via the feature serde. NOTE! this support is for Mutex, ReentrantMutex, and RwLock only; Condvar and Once are not currently supported.
18. Lock guards can be sent to other threads when the send_guard feature is enabled.

To keep these primitives small, all thread queuing and suspending functionality is offloaded to the parking lot. The idea behind this is based on the Webkit WTF::ParkingLot class, which essentially consists of a hash table mapping of lock addresses to queues of parked (sleeping) threads. The Webkit parking lot was itself inspired by Linux futexes, but it is more powerful since it allows invoking callbacks while holding a queue lock.

There are a few restrictions when using this library on stable Rust:

• The wasm32-unknown-unknown target is only fully supported on nightly with -C target-feature= atomics in RUSTFLAGS and -Zbuild-std=panic_abort,std passed to cargo. parking_lot will work mostly fine on stable, the only difference is it will panic instead of block forever if you hit a deadlock. Just make sure not to enable -C target-feature= atomics on stable as that will allow wasm to run with multiple threads which will completely break parking_lot's concurrency guarantees.

To enable nightly-only functionality, you need to enable the nightly feature in Cargo (see below).

Add this to your Cargo.toml:

[dependencies]
parking_lot = "0.12"

To enable nightly-only features, add this to your Cargo.toml instead:

[dependencies]
parking_lot = { version = "0.12", features = ["nightly"] }

The experimental deadlock detector can be enabled with the deadlock_detection Cargo feature.

To allow sending MutexGuards and RwLock*Guards to other threads, enable the send_guard option.

Note that the deadlock_detection and send_guard features are incompatible and cannot be used together.

Hardware lock elision support for x86 can be enabled with the hardware-lock-elision feature. This requires Rust 1.59 due to the use of inline assembly.

The core parking lot API is provided by the parking_lot_core crate. It is separate from the synchronization primitives in the parking_lot crate so that changes to the core API do not cause breaking changes for users of parking_lot.

The current minimum required Rust version is 1.56. Any change to this is considered a breaking change and will require a major version bump.

Licensed under either of

• Apache License, Version 2.0, (LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or https://opensource.org/licenses/MIT)

at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.