We have noticed there is significant regression in at-safepoint time with recent changes made to ShenandoahLock, more specifically this pr https://github.com/openjdk/jdk/pull/19570

We have written a reproducer which could stably reproduce the issue:
```
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Semaphore;

public class Alloc {
    static final CountDownLatch startSignal = new CountDownLatch(1);
    static final Semaphore semaphore = new Semaphore(128);
    static final int THREADS = 1024; //64 threads per CPU core, 16 cores
    static final Object[] sinks = new Object[64 * THREADS];
    static volatile boolean start;
    static volatile boolean stop;

    private static void waitOnStartSignal() {
        try {
            startSignal.await();
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
    }

    public static void main(String... args) throws Throwable {
        for (int t = 0; t < THREADS; t++) {
            int ft = t;
            new Thread(() -> work(ft * 64)).start();
        }

        Thread.sleep(1000);
        startSignal.countDown();
        Thread.sleep(30_000);
        stop = true;
    }

    public static void work(int idx) {
        waitOnStartSignal();
        while (!stop) {
            semaphore.acquireUninterruptibly();
            try {
                sinks[idx] = new byte[128];
            } catch (Throwable ex) {
                throw new RuntimeException(ex);
            } finally {
                semaphore.release();
            }
        }
    }
}
```
Run it with command-line below to list top at-safepoint times:
```
java -Xms256m -Xmx256m -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:-ShenandoahPacing -XX:-UseTLAB -Xlog:gc -Xlog:safepoint Alloc.java | grep -Po "At safepoint: \d+ ns" | grep -Po "\d+" | sort -nr
```

test with tip:
```
94069776
50993550
49321667
33903446
32291313
30587810
27759958
25080997
24657404
23874338
```

test result with revert of the https://github.com/openjdk/jdk/pull/19570
```
58428998
44410618
30788370
20636942
15986465
15307468
9686426
9432094
7473938
6854014
```