Operating system process mutual exclusion software and hardware implementation (notes)

Hardware implementation

	method	advantage	Disadvantage
Interrupt mask	Use "on/off interrupt command" to achieve,	Simple and efficient	1. Not suitable for multi-processor systems 2. Only applicable to kernel process, not applicable to user process
TesstAndSet instruction (TS instruction, TSL instruction)	The TSL instruction is implemented by hardware, and it is not allowed to be interrupted during execution, and can only be done in one go bool TSL(bool *lock){ bool old; old = *lock; *lock=true; return old; } while(TSL(&lock)); //临界区代码段 lock = false; //剩余区代码段	Simple to implement No need to check for logic vulnerabilities Suitable for multiprocessors	Not satisfied with "giving power to wait"
swap instruction	Same as above Swap(bool *a,bool *b){   bool temp;   temp=*a;   *a=*b;   *b=temp; }   bool old=true; while(old==true):   Swal(&lock,&old) //Critical section code lock=false; //Remaining area code	Same as above	Same as above

 

Software Implementation

	thought	Disadvantage
Single sign method	After the two processes have finished accessing the critical section, they will hand over the authority to use the critical section to another process. The permission of each process to enter the critical section can only be granted by another process.	Violation: Let in free, Give up and wait
Double mark first check	Set a boolean array flag[], each element in the array is used to mark the willingness of the process to enter the critical area, and true means that it wants to enter. Before entering the critical section, each process checks whether there are other processes that want to enter the critical section. If not, set its own flag to true, and then start to access the critical section.	Violation: Busy while waiting, Give up and wait
Double-mark post-inspection	An improved version of the double-mark post-inspection method. The problem with the previous algorithm is to check first and then lock, but these two operations cannot be done in one go, so the two processes may enter the critical section at the same time. So the improvement is to lock first and then check to avoid the above problems.	Violation: Let in free, Limited wait, Give up and wait
Peterson algorithm	In the double-identification post-checking method, two processes are vying to enter the critical zone, and no one is allowed to enter the critical zone in the end. Peterson's algorithm uses an idea of ​​actively letting the opponent use the critical section first.	Violation: Give up and wait