Detailed explanation of mutex locks, read-write locks, spin locks, etc. in the operating system

In operating systems, common lock types include: mutex lock, read-write lock, spin lock, condition variable, semaphore, recursion Lock (Recursive Lock), Barrier (Barrier), etc.

Mutex Lock

Mutex Lock is a common thread synchronization mechanism used to protect mutually exclusive access to shared resources in a multi-thread environment. It provides two basic operations: lock and unlock.

There are many principles and implementation methods of mutex locks. Common implementations include the use of atomic operations, mutex variables, hardware instructions, etc. The following is a brief analysis of a common mutex lock implementation principle:

1. Atomic operation implementation: Atomic operation is an uninterruptible operation that can ensure atomicity in a multi-threaded environment. In the implementation of mutex locks, a commonly used atomic operation is the compare and swap (CAS) operation. In the specific implementation, a flag bit is maintained internally in the mutex lock to indicate the status of the lock. The locking operation changes the flag bit from the unlocked state to the locked state through an atomic CAS operation. If the modification is successful, it means that the lock is acquired successfully, otherwise it needs to be retried. The unlock operation returns the flag to the unlocked state.

2. Mutually exclusive variable implementation: Mutually exclusive variables are a special type of variables that have the characteristics of atomic operations and thread synchronization. In the implementation of a mutex lock, the mutex variable is used as a flag bit to indicate the status of the lock. The locking operation obtains the value of the mutex variable through atomic testing and setting operations. If the value of the mutex variable is in the unlocked state, it is set to the locked state, indicating that the lock is acquired successfully. The unlock operation returns the value of the mutex variable to its unlocked state.

3. Hardware instruction implementation: Some processor architectures provide specific hardware instructions to support the implementation of mutex locks. These instructions can usually perform locking and unlocking operations on a single instruction level, with high performance and efficiency. These hardware instructions can ensure that the lock operation is atomic, thereby achieving thread synchronization and mutually exclusive access.

No matter which specific implementation method is used, the goal of a mutex is to protect access to shared resources so that only one thread can access the shared resources at the same time, and other threads need to wait. The locking and unlocking operations of a mutex usually need to be atomic to avoid race conditions and data inconsistencies in a multi-threaded environment.

In summary, mutex locks are a common thread synchronization mechanism used to achieve mutually exclusive access to shared resources in a multi-threaded environment. Its implementation principles can include atomic operations, mutually exclusive variables, hardware instructions, etc., aiming to ensure atomic operations on shared resources and synchronous access to threads.

Read-Write Lock

Read-Write Lock is a multi-thread synchronization mechanism used to provide better concurrency between read operations and write operations. Read-write locks allow multiple threads to perform simultaneous read operations, but require exclusive access during write operations. It contains two states: read mode and write mode.

There are many principles and implementation methods of read-write locks. The following is a brief analysis of a common implementation principle of read-write locks:

1. Read-first implementation: In the implementation of read-write locks, a counter is maintained to record the number of threads currently performing read operations. When there are no write operations, read operations can occur concurrently and the read counter is incremented. Write operations require exclusive access, so you need to wait for all read operations to end, that is, the read counter is 0, before performing a write operation. Mutually exclusive access between read and write operations can be achieved through a mutex lock.

2. How to implement write priority: In the implementation of read-write lock, a write flag bit is maintained to indicate whether there is currently a write operation. When there is no write operation, the read operation can be performed concurrently, and the read operation will not modify the shared resources, so it is safe. Write operations require exclusive access, so you need to wait for all read operations to complete and set the write flag before performing a write operation. Mutually exclusive access between read and write operations can be achieved through a mutex lock.

Regardless of the specific implementation, the goal of a read-write lock is to improve the concurrency of read operations, allow multiple threads to perform read operations at the same time, and ensure exclusive access during write operations. Read operations can be performed concurrently, but mutually exclusive access is required between read operations and write operations to ensure data consistency.

The selection and use of read-write locks depends on the specific application scenario. If read operations are frequent and concurrency requirements are high, you can choose the read-first implementation; if write operations are frequent and you need to ensure atomicity and exclusive access of write operations, you can choose the write-first implementation.

In summary, read-write lock is a multi-thread synchronization mechanism used to provide better concurrency between read operations and write operations. Its implementation principle can include two methods: read-first and write-first, aiming to achieve concurrent access for read operations and exclusive access for write operations.

Spin Lock

Spin Lock is a multi-thread synchronization mechanism used to protect critical section code to prevent multiple threads from accessing shared resources at the same time. Unlike a mutex lock, a spin lock does not cause the thread to enter a blocking state. Instead, when acquiring the lock, it continuously checks whether the lock is available until the lock is acquired.

The following is a detailed principle analysis of spin lock:

1. Initialization: The initial state of the spin lock is the unlocked state, which can be understood as the lock is in an available state.

2. Acquire the lock: When a thread wants to enter the critical section code, it will try to acquire the spin lock. If the lock is in an unlocked state, the thread can immediately acquire the lock and enter the critical section to perform operations. If the lock is in the locked state, the thread will enter the spin wait state.

3. Spin wait: When a thread finds that the spin lock has been locked by another thread, it will enter the spin wait state. During the spin wait period, the thread will continuously loop to check whether the lock status becomes available. The loop here is a busy waiting process, and the thread will continue to occupy CPU resources for checking until the lock is released.

4. Release the lock: When the thread completes the operation of the critical section, it will release the spin lock and set the lock status to the unlocked state to allow other threads to acquire the lock.

The advantage of spin lock is that it avoids thread context switching and process blocking, and is suitable for situations where the code execution time in the critical section is short and thread competition is not intense. However, spin locks also have some disadvantages. For example, during spin waiting, threads will occupy CPU resources, causing other threads to be unable to execute, which may cause a waste of resources. Therefore, spin locks are suitable for multi-threaded concurrent operations on multi-core CPUs, where threads wait for the lock for a short time.

In practical applications, the implementation of spin locks can rely on atomic operation instructions provided by the underlying hardware, or be implemented through software. The specific implementation can vary depending on the operating system and programming language.

Condition Variable

Condition variable (Condition Variable) is a multi-thread synchronization mechanism used for communication and coordination between multiple threads. It allows one or more threads to wait for a certain condition to be met before continuing execution, thereby avoiding threads' busy waiting.

The following is a detailed principle analysis of condition variables:

1. Create a condition variable: Before using a condition variable, you need to create a condition variable object. Condition variables are usually used with mutex locks, so before creating a condition variable, you also need to create a mutex lock.

2. Waiting for conditions: When a thread finds that a certain condition is not met, it can call the wait function of the condition variable to wait for the condition to be met. The wait function will cause the thread to enter the blocking state and release the previously held mutex lock, allowing other threads to enter the critical section.

3. Wake-up when a condition is met: When a thread changes the shared data so that a certain condition is met, it can call the wake-up function of the condition variable to wake up the thread waiting for the condition. The awakened thread will compete for the mutex lock again and continue execution after acquiring the lock.

4. Check the condition again: After acquiring the mutex lock, the awakened thread will check again whether the condition is met. If the condition is still not met, the thread may continue to wait or perform other operations.

The principle of condition variables is based on the waiting queue mechanism. When a thread calls the wait function, it adds itself to the wait queue of the condition variable and releases the mutex lock. When the condition is met, the wake-up function selects one or more threads from the waiting queue and notifies them to re-compete for the mutex lock.

It should be noted that the use of condition variables must be used in conjunction with mutex locks to ensure thread safety when threads wait for conditions and modify shared data. Mutexes are used to protect access to shared data, and condition variables are used to wait and wake up threads.

In practical applications, the implementation of condition variables is usually provided by the operating system, and the underlying layer uses atomic operations or other synchronization mechanisms to implement the waiting and waking up process. Differences in programming languages ​​and operating systems may lead to differences in the specific implementation of condition variables.

Semaphore

Semaphore is a multi-thread synchronization mechanism used to control access to shared resources. It controls resources through a counter and a set of waiting queues.

The following is a detailed principle analysis of semaphore:

1. Create a semaphore: Before using a semaphore, you need to create a semaphore object and initialize the counter's initial value. The counter represents the amount of available resources.

2. Acquire resources: When a thread needs to access a shared resource, it attempts to acquire a semaphore. If the counter of the semaphore is greater than 0, it means that there are available resources and the thread can continue to execute; if the counter is equal to 0, it means that there are no available resources and the thread needs to enter the waiting state.

3. Release resources: When a thread has finished using the shared resources, it needs to release the semaphore so that other threads can obtain the resources. The release operation increments the counter and may wake up a thread in the waiting queue.

4. Waiting and waking up: The waiting operation will add the thread to the semaphore's waiting queue and put the thread in a blocked state. When the semaphore's counter changes (resources are released or other threads release the semaphore), threads in the waiting queue may be awakened to compete for the semaphore again.

The principle of semaphore is based on the mechanism of counter and waiting queue. Counters are used to record the number of available resources, and waiting queues are used to save threads waiting to access resources. When acquiring the semaphore, if the counter is greater than 0, the thread can continue executing; otherwise, the thread needs to enter the waiting state. When the semaphore is released, the counter is incremented and one of the threads in the waiting queue may be awakened.

It should be noted that the semaphore does not limit only one thread to access the resource. The number of concurrent accesses can be controlled by appropriately setting the initial value of the counter.

In practical applications, the implementation of semaphores is usually provided by the operating system, and the underlying layer may use atomic operations, mutex locks, or other synchronization mechanisms to implement operations on counters and waiting queues. Different programming languages ​​and operating systems may lead to differences in the specific implementation of semaphores.

Recursive Lock

Also called a reentrant lock, it is a special type of mutex lock. Unlike ordinary mutex locks, recursive locks allow the same thread to acquire the lock multiple times without causing deadlock.
Recursive Lock is a synchronization mechanism that allows the same thread to acquire the same lock multiple times without deadlock. It allows a thread to continue acquiring a lock while holding it without being blocked.

The following is a detailed principle analysis of recursive locks:

1. Lock acquisition: When a thread attempts to acquire a recursive lock, it first checks the status of the lock. If the lock is not currently held by another thread, the thread can successfully acquire the lock and set the lock's status to belong to that thread. If the lock is already held by the current thread, the thread can also successfully acquire the lock and add 1 to the lock status counter, indicating the number of times the thread has acquired the lock multiple times.

2. Lock release: When a thread releases a recursive lock, it first checks the status of the lock. If the lock status counter is greater than 1, it means that the thread still holds the lock and only needs to decrement the counter by 1. If the lock status counter is equal to 1, it means that the thread is the last thread holding the lock and needs to clear the lock status and wake up a thread in the waiting queue.

3. Waiting and waking up: The waiting and waking up mechanism of recursive locks is similar to that of other locks. When a thread attempts to acquire a recursive lock that is already held by another thread, it is placed in a waiting queue and enters a blocking state. When the recursive lock's state is released, a thread in the waiting queue may be awakened, giving it a chance to try to acquire the lock again.

The principle of recursive locking is based on the lock status and counter mechanism. The status of the lock is used to identify the holder of the lock, and the counter is used to record the number of times a thread acquires the lock. When a thread acquires a lock, it will determine whether it can successfully acquire the lock based on the lock status and counter. When releasing the lock, the thread will use the value of the counter to determine whether it needs to actually release the lock, or just decrement the value of the counter.

The design purpose of recursive locks is to solve the need for the same thread to acquire the same lock multiple times and avoid the occurrence of deadlock. It allows a thread to continue acquiring a lock while holding it without being blocked. However, it should be noted that the use of recursive locks also needs to be cautious to avoid infinite recursion to acquire locks, causing the program to fall into an infinite loop.

Barrier

Barrier is a synchronization mechanism used to ensure that multiple threads wait synchronously at a certain point and start performing subsequent operations at the same time when conditions are met. Barriers can be used to coordinate the execution order of threads and ensure that threads perform synchronized operations on a common point.

The following is a detailed principle analysis of the barrier:

1. Creating a barrier: In a program, a barrier can be created through a specific barrier interface or function. Barriers are usually associated with a counter that tracks the number of threads that reach the barrier point.

2. Thread reaches barrier point: When a thread reaches barrier point, it notifies the barrier through barrier interface or function. The barrier increments the number of arriving threads by 1 and checks whether the predetermined number has been reached.

3. Waiting and synchronization: If the number of threads reaching the barrier point has not reached the predetermined number, the thread will be blocked and wait for other threads to reach the barrier point. Once the number of arriving threads reaches a predetermined number, the barrier triggers a signal to notify all waiting threads that they can start performing subsequent operations.

4. Subsequent operations: Once the barrier triggers the signal, all waiting threads will start performing subsequent operations at the same time. These operations can be performed in parallel because the barrier ensures that all threads wait synchronously at the same point.

The principle of barrier is based on counter and synchronization mechanism. The counter is used to track the number of threads reaching the barrier point, and the synchronization mechanism is used to block and wake up threads to ensure that threads start performing subsequent operations at the same time when the predetermined number is reached.

The role of the barrier is to coordinate the execution order of threads in a multi-threaded environment, ensuring that threads wait synchronously on a common point, and start executing subsequent operations at the same time when conditions are met. It can be used to solve synchronization problems between threads and ensure that multiple threads perform synchronized operations at a certain key point, thereby avoiding race conditions and uncertain results.