Detailed explanation of recursive locks and the difference between recursive locks and other synchronization mechanisms

What is recursive lock

Recursive lock is a multi-thread synchronization mechanism used to solve the problem of deadlock when threads acquire the same lock multiple times. In recursive locks, the same thread can acquire the same lock multiple times without causing deadlock.

Recursive locks have two main operations: lock and unlock. A thread can lock multiple times, but must unlock it multiple times to fully release the lock. Only when the number of thread unlocks is equal to the number of locks, other threads can acquire the lock.

Recursive locking works as follows:

1. When a thread requests a lock, if the lock is in an unlocked state, the thread acquires the lock and sets its status to locked, and sets the number of locks to 1.
2. If the same thread requests the lock again, the recursive lock checks whether the current thread already holds the lock. If so, the number of locks is increased by 1 and the lock continues to remain locked.
3. When the thread is unlocked, the number of locks is reduced by 1. Only when the number of locks is reduced to 0, the lock will be completely released and other threads can acquire the lock.

Recursive locks are typically used in the following situations:

1. Recursive function or method: When a recursive function or method needs to acquire the same lock in each recursive call, the recursive lock can ensure that the thread will not deadlock due to acquiring the same lock.
2. Nested critical sections: When a thread enters the same critical section again inside a critical section, recursive locks can ensure that the thread will not be blocked because it already holds the lock.

The application steps of recursive lock are as follows:

1. Create a recursive lock object.
2. In the code block that needs to be synchronized, use the lock operation of the recursive lock to acquire the lock.
3. After the code block completes execution, use the recursive lock unlock operation to release the lock.

It should be noted that each locking operation must correspond to the same number of unlocking operations, otherwise deadlock or other synchronization problems may result.

A recursive lock is a reentrant lock that allows the same thread to acquire the same lock multiple times without causing deadlock. It is usually used in scenarios such as recursive functions and nested critical sections to ensure thread safety and avoid deadlock problems.

The difference between recursive lock and mutex lock

Recursive Lock and Mutex Lock are two common thread synchronization mechanisms. They have some differences in implementation:

1. Reentrancy:

   • Recursive lock: allows the same thread to acquire the same lock multiple times without causing deadlock. Each time a thread acquires a lock, the lock counter increases, and the lock is released only when the counter reaches zero.
   • Mutex lock: The same thread is not allowed to acquire the same lock multiple times. If the same thread attempts to acquire an already held mutex again, a deadlock will result.

2. use:

   • Recursive lock: Recursive lock is mainly used to solve the problem of deadlock that may occur when the same thread acquires the same lock multiple times. It is suitable for scenarios such as recursive functions and nested critical sections.
   • Mutex lock: Mutex lock is used to protect the critical section, ensuring that only one thread can enter the critical section to execute code at the same time, preventing data competition and inconsistency caused by concurrent access.

3. Performance overhead:

   • Recursive locks: Since recursive locks need to maintain a lock counter, there will be additional performance overhead when acquiring and releasing the lock multiple times.
   • Mutex locks: Mutex locks are generally more efficient than recursive locks because they do not require the maintenance of a lock counter.

4. Deadlock risk:

   • Recursive lock: Recursive lock can avoid deadlock when the same thread acquires the same lock, but if used improperly, it may still cause other types of deadlock problems.
   • Mutex lock: Since the mutex lock does not allow the same thread to acquire the same lock multiple times, it can avoid the deadlock problem of the same thread itself.

Choosing a recursive lock or a mutex lock depends on the specific application scenario and requirements. If you need the same thread to acquire the same lock multiple times and want to avoid deadlock problems, you can choose recursive locks. If you only need to protect the consistency of the critical section and do not need the same thread to acquire the same lock multiple times, you can choose a mutex lock.

The difference between recursive lock and semaphore

Recursive Lock and Semaphore are two different thread synchronization mechanisms. They have some differences in implementation and application:

1. control target:

   • Recursive lock: Recursive lock is a binary lock that is used to solve the problem of deadlock caused by the same thread acquiring the same lock multiple times. It allows the same thread to acquire the same lock multiple times and requires the same number of unlock operations to fully release the lock.
   • Semaphore: A semaphore is a counter used to control access to shared resources. It can have an initial value, and the thread's access can be controlled by incrementing or decrementing the counter's value.

2. Concurrent access:

   • Recursive lock: Recursive lock is mainly used to solve the synchronization problem when the same thread acquires the same lock multiple times, and does not involve concurrent access between multiple threads.
   • Semaphores: Semaphores can control concurrent access to shared resources by multiple threads. By operating on the semaphore, the thread can wait for the release of resources or apply for resource permissions.

3. counter:

   • Recursive lock: Recursive lock does not maintain the concept of counter. It only cares about the status of the lock (locked or unlocked) and the thread holding the lock.
   • Semaphore: The semaphore maintains a counter that represents the number of available resources. The counter can be any non-negative integer.

4. Application scenarios:

   • Recursive lock: Recursive lock is usually used to solve the problem of the same thread acquiring the same lock multiple times in a recursive function or nested critical section.
   • Semaphores: Semaphores are usually used to control concurrent access to shared resources, limit the number of threads accessing resources at the same time, and perform synchronization and communication between threads.

Recursive locks and semaphores are two different thread synchronization mechanisms suitable for different scenarios. Recursive locks are mainly used to solve the problem of the same thread acquiring the same lock multiple times, while semaphores are used to control concurrent access to shared resources and synchronization between threads.