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1. Overview

In this tutorial, we'll explore binary semaphores and reentrant locks. Also, we'll compare them against each other to see which one is best suited in common situations.

2. What Is a Binary Semaphore?

A binary semaphore provides a signaling mechanism over the access of a single resource. In other words, a binary semaphore provides a mutual exclusion that allows only one thread to access a critical section at a time.

For that, it keeps only one permit available for access. Hence, a binary semaphore has only two states: one permit available or zero permits available.

Let's discuss a simple implementation of a binary semaphore using the Semaphore class available in Java:

Semaphore binarySemaphore = new Semaphore(1);
try {
    binarySemaphore.acquire();
    assertEquals(0, binarySemaphore.availablePermits());
} catch (InterruptedException e) {
    e.printStackTrace();
} finally {
    binarySemaphore.release();
    assertEquals(1, binarySemaphore.availablePermits());
}

Here, we can observe that the acquire method decreases the available permits by one. Similarly, the release method increases the available permits by one.

Additionally, the Semaphore class provides the fairness parameter. When set to true, the fairness parameter ensures the order in which the requesting threads acquire permits (based on their waiting time):

Semaphore binarySemaphore = new Semaphore(1, true);

3. What Is a Reentrant Lock?

A reentrant lock is a mutual exclusion mechanism that allows threads to reenter into a lock on a resource (multiple times) without a deadlock situation.

A thread entering into the lock increases the hold count by one every time. Similarly, the hold count decreases when unlock is requested. Therefore, a resource is locked until the counter returns to zero.

For instance, let's look at a simple implementation using the ReentrantLock class available in Java:

ReentrantLock reentrantLock = new ReentrantLock();
try {
    reentrantLock.lock();
    assertEquals(1, reentrantLock.getHoldCount());
    assertEquals(true, reentrantLock.isLocked());
} finally {
    reentrantLock.unlock();
    assertEquals(0, reentrantLock.getHoldCount());
    assertEquals(false, reentrantLock.isLocked());
}

Here, the lock method increases the hold count by one and locks the resource. Similarly, the unlock method decreases the hold count and unlocks a resource if the hold count is zero.

When a thread reenters the lock, it has to request for the unlock the same number of times to release the resource:

reentrantLock.lock();
reentrantLock.lock();
assertEquals(2, reentrantLock.getHoldCount());
assertEquals(true, reentrantLock.isLocked());

reentrantLock.unlock();
assertEquals(1, reentrantLock.getHoldCount());
assertEquals(true, reentrantLock.isLocked());

reentrantLock.unlock();
assertEquals(0, reentrantLock.getHoldCount());
assertEquals(false, reentrantLock.isLocked());

Similar to the Semaphore class, the ReentrantLock class also supports the fairness parameter:

ReentrantLock reentrantLock = new ReentrantLock(true);

4. Binary Semaphore vs. Reentrant Lock

4.1. Mechanism

A binary semaphore is a type of signaling mechanism, whereas a reentrant lock is a locking mechanism.

4.2. Ownership

No thread is the owner of a binary semaphore. However, the last thread that successfully locked a resource is the owner of a reentrant lock.

4.3. Nature

Binary semaphores are non-reentrant by nature, implying that the same thread can't re-acquire a critical section, else it will lead to a deadlock situation.

On the other side, a reentrant lock, by nature, allows reentering a lock by the same thread multiple times.

4.4. Flexibility

A binary semaphore provides a higher-level synchronization mechanism by allowing a custom implementation of a locking mechanism and deadlock recovery. Thus, it gives more control to the developers.

However, the reentrant lock is a low-level synchronization with a fixed locking mechanism.

4.5. Modification

Binary semaphores support operations like wait and signal (acquire and release in the case of Java's Semaphore class) to allow modification of the available permits by any process.

On the other hand, only the same thread that locked/unlocked a resource can modify a reentrant lock.

4.6. Deadlock Recovery

Binary semaphores provide a non-ownership release mechanism. Therefore, any thread can release the permit for a deadlock recovery of a binary semaphore.

On the contrary, deadlock recovery is difficult to achieve in the case of a reentrant lock. For instance, if the owner thread of a reentrant lock goes into sleep or infinite wait, it won't be possible to release the resource, and a deadlock situation will result.

5. Conclusion

In this short article, we've explored binary semaphore and reentrant locks.

First, we discussed the basic definition of a binary semaphore and a reentrant lock, along with a basic implementation in Java. Then, we compared them against each other based on a few parameters like mechanism, ownership, and flexibility.

We can certainly conclude that a binary semaphore provides a non-ownership-based signaling mechanism for mutual exclusion. At the same time, it can be further extended to provide locking capabilities with easy deadlock recovery.

On the other hand, a reentrant lock provides a reentrant mutual exclusion with owner-based locking capabilities and is useful as a simple mutex.

As usual, the source code is available over on GitHub.

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