Understanding Concurrency and Semaphores in Modern Operating Systems

Xinu Semaphores

Concurrency An important and fundamental feature in modern operating systems is concurrent execution of processes/threads. This feature is essential for the realization of multiprogramming, multiprocessing, distributed systems, and client-server model of computation. Concurrency encompasses many design issues including communication and synchronization among processes, sharing of and contention for resources. In this discussion we will look at the various design issues/problems and the wide variety of solutions available. 5/27/2025 Page 2

Xinu Resources & Critical Resources Shared resources: need mutual exclusion Tasks cooperating to complete a job Tasks contending to access a resource Tasks synchronizing Critical resources and critical region A important synchronization and mutual exclusion primitive / resource is semaphore 5/27/2025 Page 3

Critical sections and Semaphores When multiples tasks are executing there may be sections where only one task could execute at a given time: critical region or critical section There may be resources which can be accessed only be one of the processes: critical resource Semaphores can be used to ensure mutual exclusion to critical sections and critical resources 5/27/2025 Page 4

Semaphores See semaphore.h of xinu 5/27/2025 Page 5

Semaphores in exinu #include <kernel.h> #include <queue.h> /**< queue.h must define # of sem queues */ /* Semaphore state definitions */ #define SFREE 0x01 /**< this semaphore is free */ #define SUSED 0x02 /**< this semaphore is used */ /* type definition of "semaphore" */ typedef ulong semaphore; /* Semaphore table entry */ struct sentry { char state; /**< the state SFREE or SUSED */ short count; /**< count for this semaphore */ queue queue; /**< requires q.h. */ }; 6

Semaphores in exinu (contd.) extern struct sentry semtab[]; /** * isbadsem - check validity of reqested semaphore id and state * @param s id number to test; NSEM is declared to be 100 in kernel.h A system typically has a predetermined limited number of semaphores */ #define isbadsem(s) (((ushort)(s) >= NSEM) || (SFREE == semtab[s].state)) /* Semaphore function declarations */ syscall wait(semaphore); syscall signal(semaphore); syscall signaln(semaphore, short); semaphore newsem(short); syscall freesem(semaphore); syscall scount(semaphore); 7

Definition of Semaphores functions static semaphore allocsem(void); /** * newsem - allocate and initialize a new semaphore. * @param count - number of resources available without waiting. * example: count = 1 for mutual exclusion lock * @return new semaphore id on success, SYSERR on failure */ semaphore newsem(short count) { irqmask ps; semaphore sem; ps = disable(); /* disable interrupts */ sem = allocsem(); /* request new semaphore */ if ( sem != SYSERR && count >= 0 ) /* safety check */ { semtab[sem].count = count; /* initialize count */ restore(ps); /* restore interrupts */ return sem; /* return semaphore id */ } restore(ps); } 8

Semaphore: newsem contd. /** * allocsem - allocate an unused semaphore and return its index. * Scan the global semaphore table for a free entry, mark the entry * used, and return the new semaphore * @return available semaphore id on success, SYSERR on failure */ static semaphore allocsem(void) { int i = 0; while(i < NSEM) /* loop through semaphore table */ { /* to find SFREE semaphore */ if( semtab[i].state == SFREE ) { semtab[i].state = SUSED; return i; } i++; } return SYSERR; } 9

Semaphore: wait() /** * wait - make current process wait on a semaphore * @param sem semaphore for which to wait * @return OK on success, SYSERR on failure */ syscall wait(semaphore sem) { irqmask ps; struct sentry *psem; pcb *ppcb; ps = disable(); /* disable interrupts */ if ( isbadsem(sem) ) /* safety check */ { restore(ps); return SYSERR; } ppcb = &proctab[currpid]; /* retrieve pcb from process table */ psem = &semtab[sem]; /* retrieve semaphore entry */ if( --(psem->count) < 0 ) /* if requested resource is unavailable */ { ppcb->state = PRWAIT; /* set process state to PRWAIT*/ 10

Semaphore: wait() ppcb->sem = sem; /* record semaphore id in pcb */ enqueue(currpid, psem->queue); resched(); /* place in wait queue and reschedule */ } restore(ps); /* restore interrupts */ return OK; } 11

Semaphore: signal() /*signal - signal a semaphore, releasing one waiting process, and block * @param sem id of semaphore to signal * @return OK on success, SYSERR on failure */ syscall signal(semaphore sem) { irqmask ps; register struct sentry *psem; ps = disable(); /* disable interrupts */ if ( isbadsem(sem) ) /* safety check */ { restore(ps); return SYSERR; } psem = &semtab[sem]; /* retrieve semaphore entry */ if ( (psem->count++) < 0 ) /* release one process from wait queue */ { ready(dequeue(psem->queue), RESCHED_YES); } restore(ps); /* restore interrupts */ return OK; } 12

Semaphore: usage Problem 1: Create 3 tasks that each sleep for a random time and update a counter. Counter is the critical resources shared among the processes. Only one task can update the counter at a time so that counter value is correct. Problem 2: Create 3 tasks; task 1 updates the counter by 1 and then signal task 2 that updates the counter by 2 and then signals task 3 to update the counter by 3. 13

Problem 1 #include <..> //declare semaphore semaphore mutex1 = newsem(1); int counter = 0; //declare functions: proc1,proc1, proc3 ready(create((void *)proc1, INITSTK, INITPRIO, PROC1",, 2, 0, NULL), RESCHED_NO); ready(create((void *)proc2, INITSTK, INITPRIO, PROC2",, 2, 0, NULL), RESCHED_NO); ready(create((void *)proc3, INITSTK, INITPRIO, PROC3",, 2, 0, NULL), RESCHED_NO); 14

Problem 1: multi-tasks void proc1() { while (1) { sleep (rand()%10); wait(mutex1); counter++; signal(mutex1); } } void proc2() { while (1) { sleep (rand()%10); wait(mutex1); counter++; signal(mutex1); } } //similarly proc3 15

Problem 1 Task 1 Counter1 Task 2 Task 3 16

Problem 2 semaphore synch12 = newsem(0); semaphore synch23 = newsem(0); semaphore synch31 = newsem(0); ready(create((void *)proc1, INITSTK, INITPRIO, PROC1",, 2, 0, NULL), RESCHED_NO); ready(create((void *)proc2, INITSTK, INITPRIO, PROC2",, 2, 0, NULL), RESCHED_NO); ready(create((void *)proc3, INITSTK, INITPRIO, PROC3",, 2, 0, NULL), RESCHED_NO); signal(synch31); 17

Task flow void proc1() void proc2() void proc3() { while (1) { sleep (rand()%10); wait(synch31); counter++; signal(synch12); } } { while (1) { sleep (rand()%10); wait(synch12); counter++; signal(synch23); } } { while (1) { sleep(rand()%10); wait(synch23); counter++; signal(synch31); } } 18