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fix:deadlock when reentrant exclusive lock #2905
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Roiocam committed Aug 16, 2024
1 parent 9495478 commit a59045a
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Showing 2 changed files with 52 additions and 1 deletion.
11 changes: 10 additions & 1 deletion src/main/java/io/lettuce/core/protocol/SharedLock.java
Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,8 @@ class SharedLock {

private final Lock lock = new ReentrantLock();

private final ThreadLocal<Integer> sharedCnt = ThreadLocal.withInitial(() -> 0);

private volatile long writers = 0;

private volatile Thread exclusiveLockOwner;
Expand All @@ -45,6 +47,7 @@ void incrementWriters() {

if (WRITERS.get(this) >= 0) {
WRITERS.incrementAndGet(this);
sharedCnt.set(sharedCnt.get() + 1);
return;
}
}
Expand All @@ -63,6 +66,7 @@ void decrementWriters() {
}

WRITERS.decrementAndGet(this);
sharedCnt.set(sharedCnt.get() - 1);
}

/**
Expand Down Expand Up @@ -125,6 +129,11 @@ private void lockWritersExclusive() {
exclusiveLockOwner = Thread.currentThread();
return;
}
// reentrant exclusive lock
if (WRITERS.compareAndSet(this, sharedCnt.get(), -1)) {
exclusiveLockOwner = Thread.currentThread();
return;
}
}
} finally {
lock.unlock();
Expand All @@ -137,7 +146,7 @@ private void lockWritersExclusive() {
private void unlockWritersExclusive() {

if (exclusiveLockOwner == Thread.currentThread()) {
if (WRITERS.incrementAndGet(this) == 0) {
if (WRITERS.compareAndSet(this, -1, sharedCnt.get())) {
exclusiveLockOwner = null;
}
}
Expand Down
42 changes: 42 additions & 0 deletions src/test/java/io/lettuce/core/protocol/SharedLockTest.java
Original file line number Diff line number Diff line change
@@ -0,0 +1,42 @@
package io.lettuce.core.protocol;

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;

public class SharedLockTest {

@Test
public void safety_on_reentrant_lock_exclusive_on_writers() throws InterruptedException {
final SharedLock sharedLock = new SharedLock();
CountDownLatch cnt = new CountDownLatch(1);
try {
sharedLock.incrementWriters();

String result = sharedLock.doExclusive(() -> {
return sharedLock.doExclusive(() -> {
return "ok";
});
});
if ("ok".equals(result)) {
cnt.countDown();
}
} finally {
sharedLock.decrementWriters();
}

cnt.await(1, TimeUnit.SECONDS);

// verify writers won't be negative after finally decrementWriters
String result = sharedLock.doExclusive(() -> {
return sharedLock.doExclusive(() -> {
return "ok";
});
});

Assertions.assertEquals("ok", result);
}

}

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