Auto merge of rust-lang#110211 - joboet:queue_lock, r=Amanieu

Replace pthread `RwLock` with custom implementation

This is one of the last items in rust-lang#93740. I'm doing `RwLock` first because it is more self-contained and has less tradeoffs to make. The motivation is explained in the documentation, but in short: the pthread rwlock is slow and buggy and `std` can do much better. I considered implementing a parking lot, as was discussed in the tracking issue, but settled for the queue-based version because writing self-balancing binary trees is not fun in Rust...

This is a rather complex change, so I have added quite a bit of documentation to help explain it. Please point out any part that could be explained better.

~~The read performance is really good, I'm getting 4x the throughput of the pthread version and about the same performance as usync/parking_lot on an Apple M1 Max in the usync benchmark suite, but the write performance still falls way behind what usync and parking_lot achieve. I tried using a separate queue lock like what usync uses, but that didn't help. I'll try to investigate further in the future, but I wanted to get some eyes on this first.~~ [Resolved](rust-lang#110211 (comment))

r? `@m-ou-se`
CC `@kprotty`

Author: bors

std/src/lib.rs

@@ -339,12 +339,14 @@
 #![feature(portable_simd)]
 #![feature(prelude_2024)]
 #![feature(ptr_as_uninit)]
+#![feature(ptr_mask)]
 #![feature(slice_internals)]
 #![feature(slice_ptr_get)]
 #![feature(slice_range)]
 #![feature(std_internals)]
 #![feature(str_internals)]
 #![feature(strict_provenance)]
+#![feature(strict_provenance_atomic_ptr)]
 // tidy-alphabetical-end
 //
 // Library features (alloc):

std/src/sys/pal/unix/locks/mod.rs

@@ -22,10 +22,10 @@ cfg_if::cfg_if! {
         pub(crate) use futex_condvar::Condvar;
     } else {
         mod pthread_mutex;
-        mod pthread_rwlock;
         mod pthread_condvar;
+        mod queue_rwlock;
         pub(crate) use pthread_mutex::Mutex;
-        pub(crate) use pthread_rwlock::RwLock;
+        pub(crate) use queue_rwlock::RwLock;
         pub(crate) use pthread_condvar::Condvar;
     }
 }