Mutex and rwmutex in golang

Mutex The mutex , The main means of access control for shared resources , It's easy to use , Just Lock and Unlock(), Simple use does not mean simple internal implementation , We can think deeply about if Mutex What happens when you unlock repeatedly ？ Why does it show up ？

One ：Mutex data structure

src\sync\mutex.go

// A Mutex must not be copied after first use.
type Mutex struct {
	state int32
	sema  uint32
}

// A Locker represents an object that can be locked and unlocked.
type Locker interface {
	Lock()
	Unlock()
}

state： Save the relevant data of the lock For example, the state of the lock 、 How many are waiting for this lock goroutine wait

sema: Semaphore , The coroutine block waits for the semaphore , The unlocked coroutine releases the semaphore to wake up the coroutine waiting for the semaphore .

locked: Indicates whether the current lock is locked ,0 It's not locked ,1 It's locked

woken： Is there a process that has been awakened ,0： There's no synergy 1： There is a process to wake up , In the process of locking

starving: Indicates whether the current lock is hungry ,0 No hunger ,1 Starvation It means that the blocking time of some processes has exceeded 1ms

waiter: The number of coprocesses blocking waiting locks , When unlocking the co process, judge whether to release the semaphore according to this value

Two : Add unlocking process

1 Simple locking ： Suppose there is only one co process locking , No other co process interference ,locked From 0---->1, The locking process will judge locked Whether the flag bit is 0, If it is 0 Then put locked Set as 1, It represents the success of locking .

2 Locking is blocked ： Suppose when locking , The lock has been occupied by other processes (locked It's already 1), here waiter The counter will be +1, The current process will be blocked , until locked become 0 Only then can we have a chance to be awakened

3 Simply unlock ： Suppose there is no other co process blocking when unlocking ,locked from 1---->0, And there is no need to release semaphores

4 Unlock and wake up the process ： Suppose when unlocking , Yes 1 One or more processes are blocked ,locked from 1--->0, here waiter>0, So release a semaphore , Wake up a blocked process , The awakened synergetic process Locked from 0--->1, The awakened process lives a lock

3、 ... and ： The spin

When it's locked , If at present Locked It's already 1, It indicates that the lock has been occupied , The process of trying to lock does not immediately turn into blocking , It will continue to detect Locked Whether it becomes 0, This process is called spin process .

The spin time is very short , If you find that the lock has been released in the process of spinning , Then this process will immediately get all , At this time, even if other processes are awakened, the lock cannot be obtained , Can only block again .

The advantage of spin is that when locking fails, there is no need to immediately turn to blocking , There is a certain chance to get the lock , In this way, the switching of coordination process can be avoided .

The advantage of spin ： Make better use of cpu, Try to avoid co process switching .

The problem with spin ： If a lock is obtained during spin , Previously blocked processes will not get locks , If there are many locking processes and the lock is obtained by spinning every time , Then it is difficult for the blocked process to obtain the lock , Is in a state of hunger . This is the front starving, It doesn't spin in this state .

Four ：Mutex Pattern

normal Patterns and starvation Pattern

The default state is normal, In this mode , If the process fails to lock, it will not immediately turn into the blocking queue , Instead, first judge whether the spin condition is satisfied , If satisfied, it will start the spin process and try to grab the lock .