Skip to content

Commit

Permalink
fix:deadlock when reentrant exclusive lock #2905 #2879 (#2961) (#3041)
Browse files Browse the repository at this point in the history
* fix:deadlock when reentrant exclusive lock #2905

* confirm won't blocking other thread

* apply suggestions

Co-authored-by: Andy(Jingzhang)Chen <iRoiocam@gmail.com>
  • Loading branch information
tishun and Roiocam authored Nov 1, 2024
1 parent 4280728 commit 08096cf
Show file tree
Hide file tree
Showing 2 changed files with 68 additions and 2 deletions.
13 changes: 11 additions & 2 deletions 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> threadWriters = 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);
threadWriters.set(threadWriters.get() + 1);
return;
}
}
Expand All @@ -63,6 +66,7 @@ void decrementWriters() {
}

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

/**
Expand Down Expand Up @@ -121,7 +125,8 @@ private void lockWritersExclusive() {
try {
for (;;) {

if (WRITERS.compareAndSet(this, 0, -1)) {
// allow reentrant exclusive lock by comparing writers count and threadWriters count
if (WRITERS.compareAndSet(this, threadWriters.get(), -1)) {
exclusiveLockOwner = Thread.currentThread();
return;
}
Expand All @@ -137,9 +142,13 @@ private void lockWritersExclusive() {
private void unlockWritersExclusive() {

if (exclusiveLockOwner == Thread.currentThread()) {
if (WRITERS.incrementAndGet(this) == 0) {
// check exclusive look not reentrant first
if (WRITERS.compareAndSet(this, -1, threadWriters.get())) {
exclusiveLockOwner = null;
return;
}
// otherwise unlock until no more reentrant left
WRITERS.incrementAndGet(this);
}
}

Expand Down
57 changes: 57 additions & 0 deletions src/test/java/io/lettuce/core/protocol/SharedLockTest.java
Original file line number Diff line number Diff line change
@@ -0,0 +1,57 @@
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();
}

boolean await = cnt.await(1, TimeUnit.SECONDS);
Assertions.assertTrue(await);

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

Assertions.assertEquals("ok", result);

// and other writers should be passed after exclusive lock released
CountDownLatch cntOtherThread = new CountDownLatch(1);
new Thread(() -> {
try {
sharedLock.incrementWriters();
cntOtherThread.countDown();
} finally {
sharedLock.decrementWriters();
}
}).start();

await = cntOtherThread.await(1, TimeUnit.SECONDS);
Assertions.assertTrue(await);
}

}

0 comments on commit 08096cf

Please sign in to comment.