CS计算机代考程序代写 concurrency [537] Locks and Condition Variables

[537] Locks and Condition Variables

Semaphores
Questions answered in this lecture:
Review: How to implement join with condition variables?
Review: How to implement producer/consumer with condition variables?
What is the difference between semaphores and condition variables?
How to implement a lock with semaphores?
How to implement semaphores with locks and condition variables?
How to implement join and producer/consumer with semaphores?
How to implement reader/writer locks with semaphores?

CSE 2431
Systems 2
Based on slides by Andrea C. Arpaci-Dusseau
Remzi H. Arpaci-Dusseau

Concurrency Objectives
Mutual exclusion (e.g., A and B don’t run at same time)
– solved with locks

Ordering (e.g., B runs after A does something)
– solved with condition variables and semaphores

Condition Variables
wait(cond_t *cv, mutex_t *lock)
– assumes the lock is held when wait() is called
– puts caller to sleep + releases the lock (atomically)
– when awoken, reacquires lock before returning

signal(cond_t *cv)
– wake a single waiting thread (if >= 1 thread is waiting)
– if there is no waiting thread, just return, doing nothing

Join Implementation:
COrrect
void thread_exit() {
Mutex_lock(&m); // a
done = 1; // b
Cond_signal(&c); // c
Mutex_unlock(&m); // d
}
void thread_join() {
Mutex_lock(&m); // w
if (done == 0) // x
Cond_wait(&c, &m); // y
Mutex_unlock(&m); // z
}
Parent:
Child:
Parent: w x y z
Child: a b c d
Use mutex to ensure no race between interacting with state
and wait/signal

Producer/Consumer Problem
Producers generate data (like pipe writers)
Consumers grab data and process it (like pipe readers)
Use condition variables to:
make producers wait when buffers are full
make consumers wait when there is nothing to consume

void *consumer(void *arg) {
while(1) {
Mutex_lock(&m); // c1
while(numfull == 0) // c2
Cond_wait(&cond, &m); // c3
int tmp = do_get(); // c4
Cond_signal(&cond); // c5
Mutex_unlock(&m); // c6
printf(“%d\n”, tmp); // c7
}
}
void *producer(void *arg) {
for (int i=0; ivalue = initval;
}
User cannot read or write value directly after initialization
Wait or Test (sometimes P() for Dutch word)
Waits until value of sem is > 0, then decrements sem value
Signal or Increment or Post (sometimes V() for Dutch)
Increment sem value, then wake a single waiter
wait and post are atomic

Join with CV vs Semaphores
void thread_exit() {
Mutex_lock(&m); // a
done = 1; // b
Cond_signal(&c); // c
Mutex_unlock(&m); // d
}
void thread_join() {
Mutex_lock(&m); // w
if (done == 0) // x
Cond_wait(&c, &m); // y
Mutex_unlock(&m); // z
}
CVs:
void thread_exit() {
sem_post(&s)
}
void thread_join() {
sem_wait(&s);
}
sem_t s;
sem_init(&s, ???);

Semaphores:
Sem_wait(): Waits until value > 0, then decrement
Sem_post(): Increment value, then wake a single waiter

Initialize to 0 (so sem_wait() must wait…)

Equivalence Claim
Semaphores are equally powerful to Locks+CVs
– what does this mean?

One might be more convenient, but that’s not relevant

Equivalence means each can be built from the other

Proof Steps
Want to show we can do these three things:
Locks
Semaphores
CV’s
Semaphores
Locks
Semaphores
CV’s

Build Lock from Semaphore
typedef struct __lock_t {
// whatever data structs you need go here
} lock_t;

void init(lock_t *lock) {
}

void acquire(lock_t *lock) {
}

void release(lock_t *lock) {
}
Locks
Semaphores
Sem_wait(): Waits until value > 0, then decrement
Sem_post(): Increment value, then wake a single waiter

Build Lock from Semaphore
typedef struct __lock_t {
sem_t sem;
} lock_t;

void init(lock_t *lock) {
sem_init(&lock->sem, ??);
}
void acquire(lock_t *lock) {
sem_wait(&lock->sem);
}
void release(lock_t *lock) {
sem_post(&lock->sem);
}
Locks
Semaphores
1  1 thread can grab lock
Sem_wait(): Waits until value > 0, then decrement
Sem_post(): Increment value, then wake a single waiter

Building CV’s over Semaphores
Possible, but really hard to do right

Read about Microsoft Research’s attempts:
http://research.microsoft.com/pubs/64242/ImplementingCVs.pdf
CV’s
Semaphores

Build Semaphore from Lock and CV
Typedef struct {
// what goes here?

} sem_t;

Void sem_init(sem_t *s, int value) {
// what goes here?

}