CIS Operating Systems Application of Semaphores. Professor Qiang Zeng Spring 2018

Size: px

Start display at page:

Download "CIS Operating Systems Application of Semaphores. Professor Qiang Zeng Spring 2018"

Stephany Robbins
6 years ago
Views:

1 CIS Operating Systems Application of Semaphores Professor Qiang Zeng Spring 2018

2 Big picture of synchronization primitives Busy-waiting Software solutions (Dekker, Bakery, etc.) Hardware-assisted solutions (based on atomic read-modify-write instructions) Sync. Primitives Semaphore: it contains an internal counter indicating the number of resources. Binary Semaphore is a special semaphore, whose counter value can only be 0 or 1; it can be used as a mutex Blocking Monitor: language-assisted synchronization; data + procedures + lock Condition variable: guarded by a lock to wait on some event CIS 3207 Operating Systems 2

3 Previous class What is a Monitor? Monitor: a language-assisted construct that encapsulates data, procedures, and synchronization. A monitor contains a mutex lock and a queue. A process has to acquire the lock before invoking any of the procedures; if the lock is not available, the process sleeps at the queue. When a monitor procedure returns, the lock is released automatically Compiler automatically inserts lock/unlock operations upon entry/exit of each monitor procedure CIS 3207 Operating Systems 3

4 Condition Variable (CV) Note that a monitor contains a queue (waiting for the lock), while a CV has another queue (waiting to be waken up) Three operations for CVs Wait(c): It does two things The calling process is put into the wait queue of the CV Release the monitor lock (so somebody else can get in) Signal(c): it also does two things Move the calling process out of the CV s queue Move the calling process into the monitor s queue Note that : once it acquires the monitor lock, it resumes from where it was suspended (after the wait() call) Broadcast(c) Wake up all processes in the wait queue of the CV, and put then into the monitor s queue CIS 3207 Operating Systems 4

$PTHREAD_COND_INITIALIZER; Strange_withdraw(x) { pthread_mutex_lock(&m); while(account < x) // if will not work$ $pthread_cond_wait(&c, &m); account -= x; pthread_mutex_unlock(&m); } Deposit(y){ pthread_mutex_lock(&m); account +=$ Always perform CV operations within critical sections protected by a lock 2.

Always perform CV operations within critical sections protected by a lock 2.

5 How to use a Condition Variable pthread_mutex_t m = PTHREAD_MUTEX_INITIALIZER; pthread_cond_t c = PTHREAD_COND_INITIALIZER; Strange_withdraw(x) { pthread_mutex_lock(&m); while(account < x) // if will not work pthread_cond_wait(&c, &m); account -= x; pthread_mutex_unlock(&m); } Deposit(y){ pthread_mutex_lock(&m); account += y; pthread_cond_signal(&c); pthread_mutex_unlock(&m); } 1. Always perform CV operations within critical sections protected by a lock 2. Always put the wait operation in a loop (Mesa type) In order to check whether the condition is true Note the the signal(c) only hints the conditions changes; it does not imply that the condition is definitely true CIS 3207 Operating Systems 5

6 Previous class A Condition Variable has to be used with a lock. Is this true? Yes. A CV is used either with an implicit lock (e.g., the lock of a Monitor) or an explicit lock. CIS 3207 Operating Systems 6

7 Outline Restroom problem Bar problem Enforcing execution order CIS 3207 Operating Systems 7

8 Using Semaphores: The restroom problem and the bar problem

$can only be 0 or 1 Here, Binary Semaphore is used as Mutex, a blocking lock for enforcing MUTual EXclusion S = 1; // shared among customers (processes) use_restroom() { down(s); // try to enter the$

9 Binary Semaphore for the restroom problem: mutual exclusion Given a single-spot restroom, write the function for using the restroom Hint: Binary Semaphore, a special semaphore, whose counter value can only be 0 or 1 Here, Binary Semaphore is used as Mutex, a blocking lock for enforcing MUTual EXclusion S = 1; // shared among customers (processes) use_restroom() { down(s); // try to enter the restroom; = lock() Use the restroom //critical section up(s); // leave the restroom; = unlock() } CIS 3207 Operating Systems 9

Semaphore for the bar problem Capacity = 100 Many customers try to enter the bar concurrently Please write code to make sure customers<=100 Caution: a Mutex will not work

10 Semaphore for the bar problem Capacity = 100 Many customers try to enter the bar concurrently Please write code to make sure customers<=100 Caution: a Mutex will not work well; why? S = 100; // shared among processes // each process does the following down(s); // try to enter the bar Have fun; up(s); // leave the bar CIS 3207 Operating Systems 10

11 Using Semaphores: Enforcing orders

Semaphore for enforcing order Process 0 and

it as 0 S = 0; // shared // Process 0 A;

12 Semaphore for enforcing order Process 0 and Process 1 How to make sure statement A in Process 0 gets executed before statement B in Process 1 Hint: use a semaphore and initialize it as 0 S = 0; // shared // Process 0 A; up(s); // Process 1 down(s); B; CIS 3207 Operating Systems 12

a2 Hint: use two semaphores S1 = 0; // shared S2 = 0; // shared //

13 Semaphore for enforcing order Two processes: p0 and p1 How to enforce the order: p0.a1 -> p1.b1 -> p1.b2 -> p0.a2 Hint: use two semaphores S1 = 0; // shared S2 = 0; // shared // Process 0 A1; up(s1); down(s2); A2; // Process 1 down(s1); B1; B2; up(s2); CIS 3207 Operating Systems 13

14 Summary Restroom problem Bar problem Enforcing execution order CIS 3207 Operating Systems 14

Operating Systems CMPSCI 377 Spring Mark Corner University of Massachusetts Amherst

Operating Systems CMPSCI 377 Spring 2017 Mark Corner University of Massachusetts Amherst What is a Monitor? Ties data and the synchronization operations together Monitors guarantee mutual exclusion, i.e.,