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be more explicit about why adding backlinks eagerly makes sense

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joboet 2024-02-09 16:53:36 +01:00
parent 69f55de5ac
commit 3fa5a40737
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1 changed files with 7 additions and 6 deletions
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library/std/src/sys/pal/unix/locks/queue_rwlock.rs
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@ -91,11 +91,12 @@
//! //!
//! Access to the queue is controlled by the `QUEUE_LOCKED` bit, which threads //! Access to the queue is controlled by the `QUEUE_LOCKED` bit, which threads
//! try to set both after enqueuing themselves to eagerly add backlinks to the //! try to set both after enqueuing themselves to eagerly add backlinks to the
//! queue and after unlocking the lock to wake the next waiter(s). This is done //! queue, which drastically improves performance, and after unlocking the lock
//! atomically at the same time as the enqueuing/unlocking operation. The thread //! to wake the next waiter(s). This is done atomically at the same time as the
//! releasing the `QUEUE_LOCK` bit will check the state of the lock and wake up //! enqueuing/unlocking operation. The thread releasing the `QUEUE_LOCK` bit
//! waiters as appropriate. This guarantees forward-progress even if the unlocking //! will check the state of the lock and wake up waiters as appropriate. This
//! thread could not acquire the queue lock. //! guarantees forward-progress even if the unlocking thread could not acquire
//! the queue lock.
//! //!
//! ## Memory orderings //! ## Memory orderings
//! //!
@ -358,7 +359,7 @@ impl RwLock {
} else { } else {
// Otherwise, the tail of the queue is not known. // Otherwise, the tail of the queue is not known.
node.tail.set(None); node.tail.set(None);
// Try locking the queue to fully link it. // Try locking the queue to eagerly add backlinks.
next = next.map_addr(|addr| addr | QUEUE_LOCKED); next = next.map_addr(|addr| addr | QUEUE_LOCKED);
} }