SeqLock

This library provides the SeqLock type, which is a form of reader-writer lock that is heavily optimized for readers.

In certain situations, SeqLock can be two orders of magnitude faster than the standard library RwLock type. Another advantage is that readers cannot starve writers: a writer will never block even if there are readers currently accessing the SeqLock.

The only downside of SeqLock is that it only works on types that are Copy. This means that it is unsuitable for types that contains pointers to owned data.

You should instead use RwLock from the parking_lot crate if you need a reader-writer lock for types that are not Copy.

Implementation

The implementation is based on the Linux seqlock type. Each SeqLock contains a sequence counter which tracks modifications to the locked data. The basic idea is that a reader will perform the following operations:

Read the sequence counter.
Read the data in the lock.
Read the sequence counter again.
If the two sequence counter values differ, loop back and try again.
Otherwise return the data that was read in step 2.

Similarly, a writer will increment the sequence counter before and after writing to the data in the lock. Once a reader sees that the sequence counter has not changed while it was reading the data, it can safely return that data to the caller since it is known to be in a consistent state.

Example

use seqlock::SeqLock;

let lock = SeqLock::new(5);

{
    // Writing to the data involves a lock
    let mut w = lock.lock_write();
    *w += 1;
    assert_eq!(*w, 6);
}

{
    // Reading the data is a very fast operation
    let r = lock.read();
    assert_eq!(r, 6);
}

Usage

Add this to your Cargo.toml:

[dependencies]
seqlock = "0.1"

and this to your crate root:

extern crate seqlock;

To enable nightly-only features (currently just const fn constructors), add this to your Cargo.toml instead:

[dependencies]
seqlock = {version = "0.1", features = ["nightly"]}

License

Licensed under either of

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

at your option.

Contribution