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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.
## Features
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 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
2. Since they consist of just a single atomic variable, have constant
initializers and don't need destructors, these primitives can be used as
`static` global variables. The standard library primitives require
dynamic initialization and thus need to be lazily initialized with
3. Uncontended lock acquisition and release is done through fast inline
paths which only require a single atomic operation.
4. Microcontention (a contended lock with a short critical section) is
efficiently handled by spinning a few times while trying to acquire a
5. 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
6. `Condvar`, `RwLock` and `Once` work on Windows XP, unlike the standard
library versions of those types.
7. `RwLock` takes advantage of hardware lock elision on processors that
support it, which can lead to huge performance wins with many readers.
8. `RwLock` uses a task-fair locking policy, which avoids reader and writer
starvation, whereas the standard library version makes no guarantees.
9. `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.
10. `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.
11. `RwLock` supports atomically downgrading a write lock into a read lock.
12. `Mutex` and `RwLock` allow raw unlocking without a RAII guard object.
13. `Mutex<()>` and `RwLock<()>` allow raw locking without a RAII guard
14. `Mutex` and `RwLock` support [eventual fairness](
which allows them to be fair on average without sacrificing performance.
15. A `ReentrantMutex` type which supports recursive locking.
16. 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.
17. `RwLock` supports atomically upgrading an "upgradable" read lock into a
write lock.
18. 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.
19. Lock guards can be sent to other threads when the `send_guard` feature is
## The parking lot
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
but it is more powerful since it allows invoking callbacks while holding a queue
## Nightly vs stable
There are a few restrictions when using this library on stable Rust:
- You will have to use the `const_*` functions (e.g. `const_mutex(val)`) to
statically initialize the locking primitives. Using e.g. `Mutex::new(val)`
does not work on stable Rust yet.
- `RwLock` will not be able to take advantage of hardware lock elision for
readers, which improves performance when there are multiple readers.
- The `wasm32-unknown-unknown` target is only supported on nightly and requires
`-C target-feature=+atomics` in `RUSTFLAGS`.
To enable nightly-only functionality, you need to enable the `nightly` feature
in Cargo (see below).
## Usage
Add this to your `Cargo.toml`:
parking_lot = "0.11"
To enable nightly-only features, add this to your `Cargo.toml` instead:
parking_lot = { version = "0.11", features = ["nightly"] }
The experimental deadlock detector can be enabled with the
`deadlock_detection` Cargo feature.
To allow sending `MutexGuard`s and `RwLock*Guard`s to other threads, enable the
`send_guard` option.
Note that the `deadlock_detection` and `send_guard` features are incompatible
and cannot be used together.
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`.
## Minimum Rust version
The current minimum required Rust version is 1.36. Any change to this is
considered a breaking change and will require a major version bump.
## License
Licensed under either of
* Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or
at your option.
### Contribution
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.