Feature gate: #![feature(rwlock_downgrade)]

This is a tracking issue for a downgrade method for RwLock as discussed in rust-lang/libs-team#392.

The downgrade method on RwLockWriteGuard will transform a write-locked RwLock into a read-locked one.

Public API

impl<'a, T: ?Sized> RwLockWriteGuard<'a, T> {
 pub fn downgrade(s: Self) -> RwLockReadGuard<'a, T> {}
}

Steps / History

• ACP: Add a RwLock downgrade method libs-team#392
• Implementation: Rwlock downgrade #128219
  • downgrade implementation for futex.rs
  • downgrade implementation for no_threads.rs
  • downgrade implementation for queue.rs
  • downgrade implementation for solid.rs
  • downgrade implementation for teeos.rs
• Final comment period (FCP)¹
• Stabilization PR: Stabilize rwlock_downgrade library feature #143191

Unresolved Questions

• (削除) It is likely that the reader protocol for the futex implementation will need to change. See below. (削除ここまで)
• (削除) How to go about implementing downgrade for the queue.rs implementation? (削除ここまで)
• (削除) Does the solid_asp3 platform already have a downgrade method on their RwLock implementation? (削除ここまで)
• (削除) Does lock poisoning play into this at all? (削除ここまで)