concurrent execution with nested write lock would cause deadlock

deadlock.swift

import UIKit

private var imp = pthread_rwlock_t()

func wlock(_ time: Int) {
    pthread_rwlock_wrlock(&imp)
    print("[\(time)]wlock")
}

func rlock(_ time: Int) {
    pthread_rwlock_rdlock(&imp)
    print("[\(time)]rlock")
}

func unlock(_ time: Int) {
    pthread_rwlock_unlock(&imp)
    print("[\(time)]unlock")
}

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.

        pthread_rwlock_init(&imp, nil)

        DispatchQueue.concurrentPerform(iterations: 1000) { (time) in
            wlock(time)
            print("[\(time)]w1")
            
            // concurrent execution with nested write lock would cause deadlock
            // the issue won't repro with read lock.
            wlock(time)
            print("[\(time)]w2")
            unlock(time)
            
            unlock(time)
        }
    }
}

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