Synchronization and mutual exclusion mechanism spin lock mutex

A synchronization mechanism for mutual exclusion

应用场景：多线程的时候。当多个执行单元同时被执行，处理的是同一个资源时，就会导致竞态，这个时候就需要用同步或者互斥的方法来解决竞态。

线程和进程的区别：进程是拥有资源的最小单位，线程是参与调度的最小单位。

执行单元：进程、线程、SMP(对称多处理器)
资源：软件资源或者是硬件资源

并发：多个执行单元同时被执行。
竞态：多个执行单元同时被执行，处理的是同一个资源，就会导致竞态。

临界区：对资源进程操作的代码区
临界资源：可能会被多个执行单元同时访问并操作的资源

• Leads race reasons:

   1.多进程同时访问操作临界资源(进程和抢占它的进程之间会导致竞态)
   2.进程和中断
   3.对称多处理器
  

• Solution race conditions:

   同步：强调的是顺序性
   互斥：强调的是排他性
  

• Specific solutions:

   1.屏蔽中断
   2.自旋锁
   3.原子操作
   4.互斥体(互斥锁)
   5.信号量
  

Second, the spin lock

Header: linux / spinlock.h
spinlock Data Type: spinlock_t

Lock mechanism, complete mutual exclusion, and the like mutex

The spin lock;
critical section;
solution spin lock;

After the spin lock is locked, it will block out the process to seize, the process of scheduling related functions if the process occurs in a critical region (for example: sleep), equivalent to give up the execution of the CPU. At this time, the operating system to schedule other processes (process 2), 2 in the implementation process needs to be locked, but the process has not released the lock 1, 2 process will not obtain a lock, will enter the spin state, resulting in CPU will be 100% occupied. If the CPU is single core, after the sleep function, because the process 1 process 2 (enter the spin state) the CPU resources are occupied, unlock never executed, it will lead to a deadlock.

Note: Especially in the case of single-core CPU, can not appear on the process of scheduling related functions in spin lock to protect critical areas.

• Related functions:
  to define a flag flag to determine whether to open for the first time

spin_lock_init(spinlock_t *lock)
功能：初始化自旋锁
参数：
    @lock      自旋锁结构体指针   

void spin_lock(spinlock_t *lock)
功能：自旋锁上锁
参数：
    @lock    自旋锁结构体指针 
    
int spin_trylock(spinlock_t *lock)
功能：自旋锁上锁
参数：
    @lock    自旋锁结构体指针 
特点：如果上锁失败，错误返回

void spin_unlock(spinlock_t *lock)
功能：自旋锁解锁
参数：
    @lock    自旋锁结构体指针 

Spinlock initialization:

spin lock lock and unlock:

flag definitions and subtraction operation functions:

operating results:

Third, the mutex

Header: <linux / mutex.h>
Data Type: struct mutex

Mutex lock
critical section
mutex unlock

• related functions:

mutex_init(struct mutex *mutex)
    功能：初始化互斥体
    参数：
        @mutex  互斥体结构体指针
        
void  mutex_lock(struct mutex *lock)		
	功能：互斥体上锁
	参数：
		@lock   互斥体结构体指针  
	特点：互斥体上锁失败，会导致应用层进程休眠
void  mutex_unlock(struct mutex *lock)		
	功能：互斥体解锁
	参数：
		@lock   互斥体结构体指针   

Locking and unlocking the mutex:

the results: