Mutex lock, read / write lock, spin lock, pessimistic lock, and optimistic lock

The mutex

When a thread wants to access a shared resource （ Critical resources ） Before , The code that will be accessed by the thread （ A critical region ） To lock . If other threads want to access critical resources after locking and before releasing the lock , These threads will be blocked , Until it's unlocked , If one or more threads block when unlocking , Then the threads on the lock become ready , Then the first thread that becomes ready gets the right to use the resource , And lock it again , Other threads continue to block and wait .
characteristic ：
Only one thread can have a mutex at a time , Other threads have to wait
Application scenarios ：
Mutexes serialize operations , It can ensure complete and orderly operation , It is suitable for the situation that resources can only be operated by one co process , Weak concurrency ;

Read-write lock

Also called shared mutex , Read mode sharing , Write patterns are mutually exclusive . It's a bit like the read-write separation mode of database load balancing . It has three modes ： Read lock status , Write locked state and unlocked state . In short, only one thread can hold the read-write lock in write mode , But there can be multiple threads occupying the read-write lock in read mode .
When writing in lock mode , Any thread that locks it will be blocked , Until it's unlocked .
When in read lock mode , Any thread can read and lock it , But all threads that try to write lock are blocked . Until all read threads are unlocked . But when there are too many read threads , It's obviously not right that the write thread has been blocked all the time , So when a thread wants to write and lock it , Will block read and lock , Let the write lock thread lock first
characteristic ：
1） Multiple readers can read at the same time
2） Writers must be mutually exclusive （ Only one writer is allowed to write , It can't be done by readers and writers at the same time ）
3） The writer takes precedence over the reader （ Once there are writers , Then subsequent readers must wait , When waking up, priority should be given to the writer ）
Application scenarios ：
The read-write lock is suitable for reading more and writing less , Concurrency can be strong

spinlocks

Spinlocks and mutexes are very similar , The only difference is when the spinlock accesses the locked resource , Will always cycle to see if the lock is released . This is much more efficient than mutex , But it still needs to be occupied CPU. So spin locking can be applied to multinuclear CPU. But there is also a problem that when a spin lock is called recursively, it will cause deadlock . So use the spin lock carefully .
characteristic ：
Only one process can enter the critical zone at a time , Read / write lock is a special case of spin lock .

Optimism lock

This is actually an idea , When the thread goes to get the data , Think other threads won't modify data , It's not locked , However, when updating the data, the version will be used to determine whether other threads have modified the data ., If the data is modified, the update is rejected , It's called optimistic lock because there's no lock .

Pessimistic locking

When the thread goes to get the data , It is always assumed that other threads will modify data , So it locks every time it takes data , Other threads will block when they go to get data .
These two types of locks are generally used in databases , When the number of read operations of a database is much larger than the number of write operations , Using optimistic locks increases database throughput .
The mutex 、 spinlocks 、 Read-write lock , All belong to Pessimistic locking