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Rollup merge of #82596 - matklad:rwlock, r=sfackler
clarify RW lock's priority gotcha In particular, the following program works on Linux, but deadlocks on mac: ```rust use std::{ sync::{Arc, RwLock}, thread, time::Duration, }; fn main() { let lock = Arc::new(RwLock::new(())); let r1 = thread::spawn({ let lock = Arc::clone(&lock); move || { let _rg = lock.read(); eprintln!("r1/1"); sleep(1000); let _rg = lock.read(); eprintln!("r1/2"); sleep(5000); } }); sleep(100); let w = thread::spawn({ let lock = Arc::clone(&lock); move || { let _wg = lock.write(); eprintln!("w"); } }); sleep(100); let r2 = thread::spawn({ let lock = Arc::clone(&lock); move || { let _rg = lock.read(); eprintln!("r2"); sleep(2000); } }); r1.join().unwrap(); r2.join().unwrap(); w.join().unwrap(); } fn sleep(ms: u64) { std::thread::sleep(Duration::from_millis(ms)) } ``` Context: I was completely mystified by a my CI deadlocking on mac ([here](matklad/xshell#7)), until `@azdavis` debugged the issue. See a stand-alone reproduciton here: matklad/xshell#15
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