Lecture 9: Thread Synchronizations. Spring 2016 Jason Tang
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1 Lecture 9: Thread Synchronizations Spring 2016 Jason Tang Slides based upon Operating System Concept slides, Copyright Silberschatz, Galvin, and Gagne,
2 Topics Mutex Locks Semaphores Deadlocks 2
3 Mutex Locks test_and_set() and compare_and_swap() are hardware specific, not usable for the general case mutual exclusion lock (mutex): Simplest general-purpose synchronizer Protects a critical section by first acquire() a lock, and then release() the lock acquire() and release() must be atomic, usually implemented via hardware atomic instructions 3
4 Mutex Pseudocode mutex_t lock; /* some global variable */ /* producer code */ acquire(lock); counter++; release(lock); /* consumer code */ acquire(lock); if (counter == 0) { release(lock); continue; counter--; release(lock); 4
5 Mutex Implementation Often involves a boolean variable indicating if lock is available or not release() is simply setting availability to true Busy waiting: during acquire(), keep looping until lock is freed Also known as a spinlock void release(mutex_t lock) { lock.available = true; void acquire(mutex_t lock) { while (!lock.available) ; /* busy wait */ lock.available = false; 5
6 Mutex Implementation Busy waiting on a single-processor system is inefficient (why?) Mutex often implemented by enabling and disabling preemption void release(mutex_t lock) { reenable preemption void acquire(mutex_t lock) { disable preemption 6
7 Pthread Mutex Creation In Pthread, a mutex is of type pthread_mutex_t int pthread_mutex_init(pthread_mutex_t *mutex, const pthread_mutex_attr_t *mutexattr); First parameter is address to store the mutex identifier Second parameter gives options for mutex, or set to NULL for default settings Return value is always 0 On Ubuntu, man pages for pthread_mutex_init() is in the glibc-doc package (sudo apt-get install glibc-doc) 7
8 Using Pthread Mutexes int pthread_mutex_lock(pthread_mutex_t *mutex); int pthread_mutex_unlock(pthread_mutex_t *mutex); int pthread_mutex_destroy(pthread_mutex_t *mutex); Call pthread_mutex_lock() to acquire a mutex Call pthread_mutex_unlock() to release the mutex Call pthread_mutex_destroy() to free memory associated with mutex For all three of these functions, the parameter is a pointer to the mutex created by pthread_mutex_init() Return value is 0 on success, negative on error 8
9 Pthread Mutex Example Code /* producer thread */ pthread_mutex_lock(&lock); counter++; pthread_mutex_unlock(&lock); /* initialization code */ pthread_mutex_t lock; pthread_mutex_init(&lock, NULL); /* cleanup code */ pthread_mutex_destroy(&lock); /* consumer thread */ pthread_mutex_lock(&lock); if (counter == 0) { pthread_mutex_unlock(&lock); continue; counter--; pthread_mutex_unlock(lock); 9
10 Semaphores Whereas mutexes are boolean, semaphores used to count accesses When semaphore created, it is initialized to some value (often 0 or 1) wait() (originally called P()) - if value is 0, block; otherwise decrement by 1 and continue Also known as down() or take() signal() (originally called V()) - increment value by 1 Also known as raise(), post(), up(), or give() 10
11 Semaphore Implementation void signal(semaphore_t sem) { sem++; void wait(semaphore_t sem) { while (sem <= 0) ; /* busy wait */ sem--; Must guarantee that no two processes are executing signal() / wait() on same semaphore simultaneously Critical section is now the semaphore implementation On many operating systems, semaphores are implemented partially in userspace, and thus faster than mutexes 11
12 Semaphore Subtypes Counting semaphore - value ranges from 0 to infinity Binary semaphore - value can only be 0 or 1 If value is currently 0, signal() will increment it to 1 If value is currently 1, signal() is a no-op 12
13 Semaphore Pseudocode semaphore_t sem; /* some global variable */ sem = 0; /* initialize semaphore to 0 */ /* producer code */ signal(sem); /* consumer code */ wait(sem); 13
14 Pthread Semaphore Creation In Pthread, two types of semaphores (no binary semaphores): Unnamed semaphore: synchronizes threads and/or child processes int sem_init(sem_t *sem, int pshared, int value); First parameter is address to store semaphore identifier If second parameter is 1, semaphore will be shared with child processes created by fork() Third parameter is initial value for semaphore (often 0 or 1) Named semaphore: synchronizes threads and/or unrelated processes 14
15 Using Pthread Semaphores int sem_wait(sem_t *sem); int sem_post(sem_t *sem); int sem_destroy(sem_t *sem); Call sem_wait() to decrement semaphore, blocking if already 0 Call sem_post() to increment semaphore Call sem_destroy() to free memory associated with an unnamed semaphore For all three of these functions, the parameter is a pointer to the semaphore created by sem_init() Return value is 0 on success, negative on error 15
16 Pthread Semaphore Example Code /* initialization code */ sem_t sem; sem_init(&sem, 0, 0); /* producer thread */ sem_post(&sem); /* consumer thread */ sem_wait(&sem) /* cleanup code */ sem_destroy(&sem); 16
17 Deadlock Deadlock: two or more processes waiting indefinitely for each other Example 1: let S and Q be two semaphores initialized to 1 Thread A wait(s); wait(q); signal(s); signal(q); Example 2: let S and Q be initialized to 0 Thread A wait(s); signal(q); Thread B wait(q); wait(s); signal(q); signal(s); Thread B wait(q); signal(s); 17
18 Starvation Starvation: indefinite blocking; a thread is always waiting Thread A signal(sem); Thread B wait(sem); Thread C wait(sem); /* do different work */ If both threads B and C are waiting, and if OS always picks the thread B to get the semaphore, then C will never execute 18
19 Priority Inversion Scheduling problem when a lower-priority process holds a lock needed by higher-priority process Thread A (high priority) wait(sem); Thread B (low priority) post(sem); Thread C (medium priority) Thread B unable to post semaphore because OS keeps scheduling thread C Can be solved via priority-inheritance protocol Process accessing resources needed by a higher-priority process inherits the higher priority, until it has finished using those resources 19
20 Avoiding Deadlocks When acquiring multiple resources, release them in reverse order (like a stack) Use same acquisition order for all processes Thread A wait(s); wait(q); signal(q); signal(s); Thread B wait(s); wait(q); signal(q); signal(s); 20
21 Dining Philosophers Problem Five philosophers sitting around a table, alternating thinking and eating No interaction with neighbors; occasionally try to pick 2 chopsticks (one at a time) to eat from bowl Need both to eat, then release both when done 21
22 Dining Philosophers Pseudocode Declare array of 5 semaphores chopstick[5], all initialized to 1 Each philosopher i thread runs this code: wait(chopstick[i]); wait(chopstick[(i + 1) % 5]); /* eat */ signal(chopstick[(i + 1) % 5]); signal(chopstick[i]); /* think */ What is the problem with this algorithm? 22
23 Resolving Dining Philosophers Algorithm Allow at most 4 philosophers at table Allow a philosopher to pick up chopsticks only if both are available Put pick up chopsticks in a critical section Use asymmetric solution: odd-numbered philosopher pickup up left and then right chopstick, while even-numbered philosopher picks right and then left 23
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