parking_lot: A write rwlock deadlocks when there are two shared read locks present on another thread

This program deadlocks:

use parking_lot::RwLock;
use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Arc;

fn main() {
    let lock = Arc::new(RwLock::new(()));
    let running = Arc::new(AtomicBool::new(true));

    let l = lock.clone();
    let r = running.clone();
    let t0 = std::thread::spawn(move || {
        while r.load(Ordering::SeqCst) {
            let _a = l.read();
            let _b = l.read();
        }
    });

    let l = lock.clone();
    let r = running.clone();
    let t1 = std::thread::spawn(move || {
        while r.load(Ordering::SeqCst) {
            let _a = l.write();
        }
    });

    std::thread::sleep(std::time::Duration::from_millis(1000));

    running.store(false, Ordering::SeqCst);
    t0.join().unwrap();
    t1.join().unwrap();
}

If you comment out let _b = l.read(); then everything is fine.

Exactly the same program which uses RwLock from std doesn’t deadlock:

use std::sync::RwLock;
use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Arc;

fn main() {
    let lock = Arc::new(RwLock::new(()));
    let running = Arc::new(AtomicBool::new(true));

    let l = lock.clone();
    let r = running.clone();
    let t0 = std::thread::spawn(move || {
        while r.load(Ordering::SeqCst) {
            let _a = l.read().unwrap();
            let _b = l.read().unwrap();
        }
    });

    let l = lock.clone();
    let r = running.clone();
    let t1 = std::thread::spawn(move || {
        while r.load(Ordering::SeqCst) {
            let _a = l.write().unwrap();
        }
    });

    std::thread::sleep(std::time::Duration::from_millis(1000));

    running.store(false, Ordering::SeqCst);
    t0.join().unwrap();
    t1.join().unwrap();
}

Stack traces of the deadlock:

#0  0x00007f152040ce9d in syscall () from /usr/lib/libc.so.6
#1  0x0000561815ecab20 in parking_lot_core::thread_parker::imp::ThreadParker::futex_wait (self=0x7f151fd90660, ts=...) at /home/kou/.cargo/git/checkouts/parking_lot-10afde9c7055db59/fa294cd/core/src/thread_parker/linux.rs:111
#2  0x0000561815eca84a in <parking_lot_core::thread_parker::imp::ThreadParker as parking_lot_core::thread_parker::ThreadParkerT>::park (self=0x7f151fd90660)
    at /home/kou/.cargo/git/checkouts/parking_lot-10afde9c7055db59/fa294cd/core/src/thread_parker/linux.rs:65
#3  0x0000561815ec8d3d in parking_lot_core::parking_lot::park::{{closure}} (thread_data=0x7f151fd90640) at /home/kou/.cargo/git/checkouts/parking_lot-10afde9c7055db59/fa294cd/core/src/parking_lot.rs:611
#4  0x0000561815ec89b4 in parking_lot_core::parking_lot::with_thread_data (f=...) at /home/kou/.cargo/git/checkouts/parking_lot-10afde9c7055db59/fa294cd/core/src/parking_lot.rs:183
#5  parking_lot_core::parking_lot::park (key=94661465940480, validate=..., before_sleep=..., timed_out=..., park_token=..., timeout=...) at /home/kou/.cargo/git/checkouts/parking_lot-10afde9c7055db59/fa294cd/core/src/parking_lot.rs:576
#6  0x0000561815ec1341 in parking_lot::raw_rwlock::RawRwLock::lock_common (self=0x5618170d2200, timeout=..., token=..., try_lock=..., validate_flags=8)
    at /home/kou/.cargo/git/checkouts/parking_lot-10afde9c7055db59/fa294cd/src/raw_rwlock.rs:1103
#7  0x0000561815ec00f4 in parking_lot::raw_rwlock::RawRwLock::lock_shared_slow (self=0x5618170d2200, recursive=false, timeout=...) at /home/kou/.cargo/git/checkouts/parking_lot-10afde9c7055db59/fa294cd/src/raw_rwlock.rs:707
#8  0x0000561815ebaf52 in <parking_lot::raw_rwlock::RawRwLock as lock_api::rwlock::RawRwLock>::lock_shared (self=0x5618170d2200) at /home/kou/.cargo/git/checkouts/parking_lot-10afde9c7055db59/fa294cd/src/raw_rwlock.rs:109
#9  0x0000561815ebb6d6 in lock_api::rwlock::RwLock<R,T>::read (self=0x5618170d2200) at /home/kou/.cargo/git/checkouts/parking_lot-10afde9c7055db59/fa294cd/lock_api/src/rwlock.rs:337
#10 0x0000561815ebf8e1 in tst2::main::{{closure}} () at src/main.rs:14

#0  0x00007f152040ce9d in syscall () from /usr/lib/libc.so.6
#1  0x0000561815ecab20 in parking_lot_core::thread_parker::imp::ThreadParker::futex_wait (self=0x7f151fb8f660, ts=...) at /home/kou/.cargo/git/checkouts/parking_lot-10afde9c7055db59/fa294cd/core/src/thread_parker/linux.rs:111
#2  0x0000561815eca84a in <parking_lot_core::thread_parker::imp::ThreadParker as parking_lot_core::thread_parker::ThreadParkerT>::park (self=0x7f151fb8f660)
    at /home/kou/.cargo/git/checkouts/parking_lot-10afde9c7055db59/fa294cd/core/src/thread_parker/linux.rs:65
#3  0x0000561815ec9d99 in parking_lot_core::parking_lot::park::{{closure}} (thread_data=0x7f151fb8f640) at /home/kou/.cargo/git/checkouts/parking_lot-10afde9c7055db59/fa294cd/core/src/parking_lot.rs:611
#4  0x0000561815ec87ff in parking_lot_core::parking_lot::with_thread_data (f=...) at /home/kou/.cargo/git/checkouts/parking_lot-10afde9c7055db59/fa294cd/core/src/parking_lot.rs:183
#5  parking_lot_core::parking_lot::park (key=94661465940481, validate=..., before_sleep=..., timed_out=..., park_token=..., timeout=...) at /home/kou/.cargo/git/checkouts/parking_lot-10afde9c7055db59/fa294cd/core/src/parking_lot.rs:576
#6  0x0000561815ec0b3a in parking_lot::raw_rwlock::RawRwLock::wait_for_readers (self=0x5618170d2200, timeout=..., prev_value=0) at /home/kou/.cargo/git/checkouts/parking_lot-10afde9c7055db59/fa294cd/src/raw_rwlock.rs:1001
#7  0x0000561815ebfe3d in parking_lot::raw_rwlock::RawRwLock::lock_exclusive_slow (self=0x5618170d2200, timeout=...) at /home/kou/.cargo/git/checkouts/parking_lot-10afde9c7055db59/fa294cd/src/raw_rwlock.rs:632
#8  0x0000561815ebb09d in <parking_lot::raw_rwlock::RawRwLock as lock_api::rwlock::RawRwLock>::lock_exclusive (self=0x5618170d2200) at /home/kou/.cargo/git/checkouts/parking_lot-10afde9c7055db59/fa294cd/src/raw_rwlock.rs:73
#9  0x0000561815ebb706 in lock_api::rwlock::RwLock<R,T>::write (self=0x5618170d2200) at /home/kou/.cargo/git/checkouts/parking_lot-10afde9c7055db59/fa294cd/lock_api/src/rwlock.rs:369
#10 0x0000561815ebf9c2 in tst2::main::{{closure}} () at src/main.rs:22

Originally I was investigating why my upgradable rwlocks deadlock, and I wanted to create an issue with the following snippet which also deadlocks:

use parking_lot::lock_api::RwLockUpgradableReadGuard;
use parking_lot::RwLock;
use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Arc;

fn main() {
    let lock = Arc::new(RwLock::new(()));
    let running = Arc::new(AtomicBool::new(true));

    let l = lock.clone();
    let r = running.clone();
    let t0 = std::thread::spawn(move || {
        while r.load(Ordering::SeqCst) {
            let _a = l.read();
            let _b = l.read();
        }
    });

    let l = lock.clone();
    let r = running.clone();
    let t1 = std::thread::spawn(move || {
        while r.load(Ordering::SeqCst) {
            let a = l.upgradable_read();
            let _b = RwLockUpgradableReadGuard::upgrade(a);
        }
    });

    std::thread::sleep(std::time::Duration::from_millis(1000));

    running.store(false, Ordering::SeqCst);
    t0.join().unwrap();
    t1.join().unwrap();
}