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# Summary ch5

1. **Race Condition:**A situation where several processes access and manipulate the same data concurrently and the outcome of the execution depends on the particular order in which the access takes place. 
2. **Critical Section:**

   ![](/files/-Lpv9P8BO6468KL8XghA)
3. **Solution to Critical Section Problem:**  

   (1)**Mutual Exclusion** - If process Pi is executing in its critical section, then no other processes can be executing in their critical sections  

   (2)**Progress** – When no process is in a critical section, any process that requests entry to its critical section must be permitted to enter without delay.   

   (3)**Bounded Waiting** -  A bound must exist on the number of times that other processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is granted
4. **Mutual Exclusion: Hardware Support**  

   (1)**Interrupt Disabling:**  

   A process runs until it invokes an operating system service or until it is interrupted;  

   Disabling interrupts guarantees mutual exclusion  

   (2)**Disadvantages:**  

   Processor is limited in its ability to interleave programs;  

   Will not work in multiprocessor architecture.
5. **Mutual Exclusion Machine-Instruction:**  

   **Advantages:**  

   (1)Applicable to any number of processes on a single processor  

   (2)Processes on multiple processors(as long as processors share main memory)  

   (3)simple and therefore easy to verify  

   (4)It can be used to support multiple critical sections  

   **Disadvantages:**  

   (1)Busy-waiting consumes processor time  

   (2)Starvation is possible when a process leaves a critical section and more than one process is waiting 
6. **Semaphore** is a variable that has an integer value  

   (1)May be initialized to a nonnegative number  

   (2)Wait operation decrements the semaphore value  

   (3)Signal operation increments semaphore value
7. **Mutual Exclusion Using Semaphores:**  

   ![](/files/-Lpv9P8DP-rsMTkJYKjZ)