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Synchronization

Threads communicate to ensure consistency If not: race condition

(non-deterministic result) Accomplished by synchronization

operations

How to write concurrent code How to implement synchronization

operations

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Synchronization – Motivation

“The too much milk problem”

Model of need to synchronize activities

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Synchronization Terminology

Mutual exclusion (“mutex”) – prevents multiple threads from entering

Critical section – regions of code that modify or access shared variables

– code only one thread can execute at a time

Lock– mechanism for mutual exclusion Lock on entering critical section, accessing

shared data Unlock when complete Wait if locked

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Solving the Too Much Milk Problem

Correctness properties Safety: “nothing bad happens” Progress: “something good

eventually happens”

First: use atomic loads & stores as building blocks “Leave a note” (lock) “Remove a note” (unlock) “Don’t buy milk if there’s a note” (wait)

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Too Much Milk: Solution 1

thread A

if (no milk && no note)

leave note

buy milk

remove note

thread B

if (no milk && no note)

leave note

buy milk

remove note

Does this work?too much milk

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Too Much Milk: Solution 2

thread A

leave note A

if (no note B)

if (no milk)

buy milk

remove note A

thread B

leave note B

if (no note A)

if (no milk)

buy milk

remove note B

Idea: use labeled notes

oops – no milk

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Language Support

Synchronization complicated Better way – provide language-level

support Higher-level approach Hide gory details in runtime system

Increasingly high-level approaches: Locks, Atomic Operations Semaphores – generalized locks Monitors – tie shared data to

synchronization

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Locks

Provide mutual exclusion to shared data via two atomic routines Lock::Acquire – wait for lock, then take

it Lock::Release – unlock, wake up waiters

Rules: Acquire lock before accessing shared

data Release lock afterwards Lock – initially released

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Too Much Milk: Locks

Clean, symmetric - but how do we implement it?

thread A

Lock.acquire()

if (no milk)

buy milk

Lock.release()

thread B

Lock.acquire()

if (no milk)

buy milk

Lock.release()

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Implementing Locks

Requires hardware support (in general)

Can build on atomic operations: Disable interrupts

Uniprocessors only Test & Set, Compare & Swap

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Disabling Interrupts

Prevent scheduler from switching threads in middle of critical sections Ignores quantum expiration (timer

interrupt) No handling I/O operations

(Don’t make I/O calls in critical section!)

To ensure current sequence of instructions run atomically

Drawback?

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Atomic Read-Write-Modify Instructions Atomically read old value, write

new value

Examples: Test & Set (most arch) Exchange (x86) Compare & Swap (68K, Sparc)

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Implementing Locks: Test & Set

int testset (int value) { int old = value; value = 1; return old;}

pseudo-code: red = atomic

class Lock { private int value; Lock() { value = 0; } void acquire() {…} void release() {…}}

void acquire() { while (testset(value)) {}}

void release() { value = 0;}

Effect of testset(value) when value = 0? or 1?

acquire – wait until lock released, then take it

release – release lock value: 1 (locked); 0

(unlocked)

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Busy Waiting (“Spinning”)

What’s wrong with this implementation? CPU utilization? Different

priorities?

void acquire() { while (testset(value))

{}}

void release() { value = 0;}

spin-lock

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Minimizing Busy Waiting

Can’t implement locks with test & set without any waiting (w/o disabling interrupts) Add queue to lock and sleep: blocking

lockvoid acquire() { while (1) { if (testset(value)) { put thread on queue, sleep } else { break; } } }

void release() { value = 0; wake up threads;}

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Locks in POSIX

pthread_mutex_t my_mutex;pthread_mutex_init(&my_mutex);// also: pthread_mutex_t my_mutex = PTHREAD_MUTEX_INITIALIZER;...pthread_mutex_lock(&my_mutex);x = x + 1; /* This is the critical section */pthread_mutex_unlock(&my_mutex);...pthread_mutex_destroy(&my_mutex);

pthread_mutex_t my_mutex;pthread_mutex_init(&my_mutex);// also: pthread_mutex_t my_mutex = PTHREAD_MUTEX_INITIALIZER;...pthread_mutex_lock(&my_mutex);x = x + 1; /* This is the critical section */pthread_mutex_unlock(&my_mutex);...pthread_mutex_destroy(&my_mutex);

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Locks in POSIX: Example

Reader-writer problem: Share a buffer which holds one item (an

integer) A single reader and writer Reader: only read when there is item in

buffer Writer: only write when there is space in

buffer

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Locks in POSIX: Example - II#include <pthread.h>

#include <stdio.h>

int buffer_has_item = 0;

int buffer;

pthread_mutex_t mutex;

void * reader_function(void *)

{

while(1){

pthread_mutex_lock( &mutex );

if ( buffer_has_item == 1) {

printf("reader consumes one item: %d.\n", buffer);

buffer_has_item = 0;

}

pthread_mutex_unlock( &mutex );

}

}

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Locks in POSIX: Example - IIIvoid writer_function(void)

{

while(1) {

pthread_mutex_lock( &mutex );

if ( buffer_has_item == 0 ){

buffer = rand()*100;

printf("writer produces one item: %d\n", buffer);

buffer_has_item = 1;

}

pthread_mutex_unlock( &mutex );

}

}

void main()

{

pthread_t reader;

pthread_mutex_init(&mutex, NULL);

pthread_create( &reader, NULL, reader_function,NULL);

writer_function();

}

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Locks as Synch Primitive

+ Locks provide mutual exclusion+ Only one thread enters section at a time+ Simplifies writing concurrent programs

- Low-level- Can complicate coding

- e.g., “allow at most n threads to access resource”

What are some alternatives?

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Locks & Semaphores

Implementing locks Semaphores Monitors

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Semaphores

What’s a “semaphore” anyway?

A visual system for sending information by means of two flags that are held one in each hand, using an alphabetic code based on the position of the signaler's arms.

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Semaphores

What’s a “semaphore” anyway?

A visual signaling apparatus with flags, lights, or mechanically moving arms.

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Semaphores in CS

Computer science: Dijkstra (1965)

A non-negative integer counter with atomic increment & decrement.  Blocks rather than going negative. Higher-level than locks but not too high level

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Semaphores: Key Concepts

P(sem), a.k.a. wait = decrement counter

If sem = 0, block until greater than zero

P = “prolagen” (proberen te verlagen, “try to decrease”)

In Holland the good Dr. DijkstraTook a break from a walk on his bijkstra

And said: "Which shall it be?Take a P or a V?

For the two seem to me quite alijkstra!"

V(sem), a.k.a. signal = increment counter

Wake 1 waiting process

V = “verhogen” (“increase”)

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Implementing Semaphores

class Semaphore { private int value; private Queue q; Semaphore(int v) { value = v; }}

void wait() { if (value > 0) { value = value – 1; }

if (value == 0) { add this process to Q; sleep(); } }

void signal() { value = value + 1; if (anyone on Q) { remove P from Q; wakeup(P); } }

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Variants of Semaphores Binary semaphore

just two values (0 or 1), typically initial value 1 (“free”)

Counting semaphore useful when units of resource are

available initialized to number of resources thread can access as long as one

unit available

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Binary Semaphores: Example

“too much milk” with locks

thread A

Lock.acquire()

if (no milk)

buy milk

Lock.release()

thread B

Lock.acquire()

if (no milk)

buy milk

Lock.release()

thread A

sem.wait()

if (no milk)

buy milk

sem.signal()

thread B

sem.wait()

if (no milk)

buy milk

sem.signal()

“too much milk” with binary semaphores(initially 1)

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Binary Semaphore: Example

More flexible than locks! By initializing semaphore to 0,

threads can wait for an event to occur

thread A

// wait for thread B

sem.wait();// do stuff …

thread B

// do stuff, then

// wake up A

sem.signal();

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Counting Semaphores: Example

Controlling resources: Allow threads to use at most 5 files simultaneously

Initialize to 5

thread A

sem.wait();// use a file

sem.signal();

thread B

sem.wait();// use a file

sem.signal();

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Semaphore in POSIX

sem_t sem;sem_init(&sem, int pshared, unsigned value);

sem_wait(&sem);sem_post(&sem);

sem_destroy(&sem);

sem_t sem;sem_init(&sem, int pshared, unsigned value);

sem_wait(&sem);sem_post(&sem);

sem_destroy(&sem);

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Solving Reader-writer Problem Using Semaphore

Reader-writer problem: Share a buffer which holds one item (an

integer) A single reader and writer Reader: only read when there is item in

buffer Writer: only write when there is space

in buffer How to make the writer enter the

critical section first? How to make the reader & writer

alternate to enter critical section?

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Semaphore in Linux: Solving Reader-writer Problem#include <stdio.h>

#include <stdlib.h>

#include <pthread.h>

#include <semaphore.h>

sem_t readers_turn, writers_turn; /* semaphore declaration */

int buffer, loopcnt = 5;

void * reader_function(void *)

{

int i;

for (i=0; i<loopcnt; i++) {

sem_wait( &readers_turn );

printf("reader consumes one item: %d.\n", buffer);

sem_post( &writers_turn );

}

}

void writer_function(void) {

int i;

for (i=0; i<loopcnt; i++) {

sem_wait( &writers_turn );

buffer = rand();

printf("writer produces one item: %d\n", buffer);

sem_post( &readers_turn );

}

}

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Semaphore in Linux: Solving Reader-writer Problem - IIint main()

{

pthread_t reader;

if (sem_init(&readers_turn, 0, 0) < 0) {

perror("sem_init");

exit(1);

}

if (sem_init(&writers_turn, 0, 1) < 0) {

perror("sem_init");

exit(1);

}

if(pthread_create( &reader, NULL, reader_function, NULL) != 0) {

perror("pthread_create");

exit(1);

}

writer_function();

sem_destroy(&readers_turn);

sem_destroy(&writers_turn);

return 1;

}

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Summary

Implementing locks Test & Set Spin locks, blocking locks

Semaphores Generalization of locks Binary, counting (“Dijkstra-style”) Useful for:

Controlling resources Waiting on events