a15ac30162
Replace RwLock by a futex based one on Linux This replaces the pthread-based RwLock on Linux by a futex based one. This implementation is similar to [the algorithm](https://gist.github.com/kprotty/3042436aa55620d8ebcddf2bf25668bc) suggested by `@kprotty,` but modified to prefer writers and spin before sleeping. It uses two futexes: One for the readers to wait on, and one for the writers to wait on. The readers futex contains the state of the RwLock: The number of readers, a bit indicating whether writers are waiting, and a bit indicating whether readers are waiting. The writers futex is used as a simple condition variable and its contents are meaningless; it just needs to be changed on every notification. Using two futexes rather than one has the obvious advantage of allowing a separate queue for readers and writers, but it also means we avoid the problem a single-futex RwLock would have of making it hard for a writer to go to sleep while the number of readers is rapidly changing up and down, as the writers futex is only changed when we actually want to wake up a writer. It always prefers writers, as we decided [here](https://github.com/rust-lang/rust/issues/93740#issuecomment-1070696128). To be able to prefer writers, it relies on futex_wake to return the number of awoken threads to be able to handle write-unlocking while both the readers-waiting and writers-waiting bits are set. Instead of waking both and letting them race, it first wakes writers and only continues to wake the readers too if futex_wake reported there were no writers to wake up. r? `@Amanieu` |
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| benches | ||
| primitive_docs | ||
| src | ||
| tests | ||
| build.rs | ||
| Cargo.toml | ||