Process Synchronization and Deadlock

7/30/2019 Process Synchronization and Deadlock

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Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition

Chapter 6: ProcessSynchronization


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6.2 Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition

Module 6: Process Synchronization

Background

The Critical-Section Problem Semaphores

Classic Problems of Synchronization


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Objectives

To introduce the critical-section problem, whose solutions can be used to ensure the consistency ofshared data

To present both software and hardware solutions of the critical-section problem


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Background

Concurrent access to shared data may result in data inconsistency

Maintaining data consistency requires mechanisms to ensure the orderly execution of cooperatingprocesses

Suppose that we wanted to provide a solution to the consumer-producer problem that fills allthe buffers.We can do so by having an integer countthat keeps track of the number of full buffers. Initially, count isset to 0. It is incremented by the producer after it produces a new buffer and is decremented by theconsumer after it consumes a buffer.


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Producer

while (true) {

/* produce an item and put in nextProduced */

while (counter == BUFFER_SIZE)

; // do nothing

buffer [in] = nextProduced;

in = (in + 1) % BUFFER_SIZE;

counter++;

}



Consumer

while (true) {while (counter == 0)

; // do nothing

nextConsumed = buffer[out];

out = (out + 1) % BUFFER_SIZE;

counter--;

/* consume the item in nextConsumed

}



Race Condition

counter++ could be implemented as

register1 = counterregister1 = register1 + 1counter = register1

counter-- could be implemented as

register2 = counter

register2 = register2 - 1count = register2

Consider this execution interleaving with count = 5 initially:

S0: producer execute register1 = counter {register1 = 5}S1: producer execute register1 = register1 + 1 {register1 = 6}S2: consumer execute register2 = counter {register2 = 5}S3: consumer execute register2 = register2 - 1 {register2 = 4}

S4: producer execute counter = register1 {count = 6 }S5: consumer execute counter = register2 {count = 4}



Critical Section Problem

Consider system of n processes {p0, p1, pn-1}

Each process has critical section segment of code

Process may be changing common variables, updating table, writing file, etc

When one process in critical section, no other may be in its critical section

Critical section problem is to design protocol to solve this

Each process must ask permission to enter critical section in entry section, may follow critical section with exisection, then remainder section

Especially challenging with preemptive kernels



Critical Section

General structure of process pi is



Solution to Critical-Section Problem

1. Mutual Exclusion - If process Pi is executing in its critical section, then no other processes can beexecuting in their critical sections

2. Progress - If no process is executing in its critical section and there exist some processes that wish to entertheir critical section, then the selection of the processes that will enter the critical section next cannot bepostponed indefinitely

3. Bounded Waiting - A bound must exist on the number of times that other processes are allowed to entertheir critical sections after a process has made a request to enter its critical section and before thatrequest is granted



Semaphore

Synchronization tool

Semaphore S integer variable Two standard operations modify S: wait() and signal()

Originally called P() andV()

Less complicated

Can only be accessed via two indivisible (atomic) operations

wait (S) {

while S



Semaphore asGeneral Synchronization Tool

Countingsemaphore integer value can range over an unrestricted domain

Binary semaphore integer value can range only between 0and 1; can be simpler to implement

Also known as mutex locks

Can implement a counting semaphore S as a binary semaphore

Provides mutual exclusion

Semaphore mutex; // initialized to 1

do {

wait (mutex);

// Critical Section

signal (mutex);

// remainder section

} while (TRUE);


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Semaphore Implementation

Must guarantee that no two processes can execute wait () and signal () on the same semaphore at thesame time

Thus, implementation becomes the critical section problem where the wait and signal code are placed inthe crtical section

Could now have busy waitingin critical section implementation

But implementation code is short

Little busy waiting if critical section rarely occupied

Note that applications may spend lots of time in critical sections and therefore this is not a good solution

Semaphore Implementation


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Semaphore Implementationwith no Busy waiting

With each semaphore there is an associated waiting queue

Each entry in a waiting queue has two data items: value (of type integer)

pointer to next record in the list

Two operations:

block place the process invoking the operation on the appropriate waiting queue

wakeup remove one of processes in the waiting queue and place it in the ready queue

Semaphore Implementation with


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Semaphore Implementation withno Busy waiting (Cont.)

Implementation of wait:

wait(semaphore *S) {

S->value--;

if (S->value < 0) {

add this process to S->list;

block();

}

}

Implementation of signal:

signal(semaphore *S) {

S->value++;

if (S->value list;

wakeup(P);

}

}


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Deadlock and Starvation

Deadlock two or more processes are waiting indefinitely for an event that can be caused by only one ofthe waiting processes

Let S and Q be two semaphores initialized to 1P0 P1

wait (S); wait (Q);

wait (Q); wait (S);

. .

. .

. .

signal (S); signal (Q);

signal (Q); signal (S);

Starvation indefinite blocking

A process may never be removed from the semaphore queue in which it is suspended

Priority Inversion Scheduling problem when lower-priority process holds a lock needed by higher-

priority process Solved via priority-inheritance protocol


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Classical Problems of Synchronization

Classical problems used to test newly-proposed synchronization schemes

Bounded-Buffer Problem

Readers and Writers Problem

Dining-Philosophers Problem


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Bounded-Buffer Problem

Nbuffers, each can hold one item

Semaphore mutex initialized to the value 1

Semaphore full initialized to the value 0

Semaphore empty initialized to the value N


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Bounded Buffer Problem (Cont.)

The structure of the producer process

do {

// produce an item in nextp

wait (empty);

wait (mutex);

// add the item to the buffer

signal (mutex);

signal (full);

} while (TRUE);


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Bounded Buffer Problem (Cont.)

The structure of the consumer process

do {

wait (full);

wait (mutex);

// remove an item from buffer to nextc

signal (mutex);

signal (empty);

// consume the item in nextc

} while (TRUE);


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Readers-Writers Problem

A data set is shared among a number of concurrent processes

Readers only read the data set; they do not perform any updates

Writers can both read and write

Problem allow multiple readers to read at the same time

Only one single writer can access the shared data at the same time

Several variations of how readers and writers are treated all involve priorities

Shared Data

Data set

Semaphore mutex initialized to 1

Semaphore wrt initialized to 1

Integer readcount initialized to 0


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Readers-Writers Problem (Cont.)

The structure of a writer process

do {

wait (wrt) ;

// writing is performed

signal (wrt) ;

} while (TRUE);


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Readers-Writers Problem (Cont.)

The structure of a reader process

do {

wait (mutex) ;

readcount ++ ;

if (readcount == 1)

wait (wrt) ;

signal (mutex)

// reading is performed

wait (mutex) ;

readcount - - ;

if (readcount == 0)

signal (wrt) ;

signal (mutex) ;

} while (TRUE);


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Readers-Writers Problem Variations

Firstvariation no reader kept waiting unless writer has permission to use shared object

Secondvariation once writer is ready, it performs write asap

Both may have starvation leading to even more variations

Problem is solved on some systems by kernel providing reader-writer locks

Di i Phil h P bl


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Dining-Philosophers Problem

Philosophers spend their lives thinking and eating

Dont interact with their neighbors, occasionally try to pick up 2 chopsticks (one at a time) to eat

from bowl

Need both to eat, then release both when done

In the case of 5 philosophers

Shared data

Bowl of rice (data set)

Semaphore chopstick [5] initialized to 1

Di i Phil h P bl Al ith


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Dining-Philosophers Problem Algorithm

The structure of Philosopher i:

do {wait ( chopstick[i] );

wait ( chopStick[ (i + 1) % 5] );

// eat

signal ( chopstick[i] );

signal (chopstick[ (i + 1) % 5] );

// think

} while (TRUE);

What is the problem with this algorithm?

Th D dl k P bl


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The Deadlock Problem

A set of blocked processes each holding a resource and

waiting to acquire a resource held by another process in theset

Example

System has 2 disk drives

P1 and P2 each hold one disk drive and each needs another one Example

semaphores A and B, initialized to 1

P0 P1

wait (A); wait(B)

wait (B); wait(A)

B id C i E l


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Bridge Crossing Example

Traffic only in one direction

Each section of a bridge can be viewed as a resource

If a deadlock occurs, it can be resolved if one car backs up (preemptresources and rollback)

Several cars may have to be backed up if a deadlock occurs

Starvation is possible

Note Most OSes do not prevent or deal with deadlocks

System Model


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System Model

Resource types R1, R2, . . ., RmCPU cycles, memory space, I/O devices

Each resource type Ri has Wi instances.

Each process utilizes a resource as follows:

request

use

release

Deadlock Characterization


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Deadlock Characterization

Mutual exclusion: only one process at a time can use a resource

Hold and wait: a process holding at least one resource is waiting toacquire additional resources held by other processes

No preemption: a resource can be released only voluntarily by theprocess holding it, after that process has completed its task

Circular wait: there exists a set {P0, P1, , Pn} of waiting processes suchthat P0 is waiting for a resource that is held by P1, P1 is waiting for aresource that is held by

P2, , Pn1 is waiting for a resource that is held by Pn, and Pn is waiting fora resource that is held by P0.

Deadlock can arise if four conditions hold simultaneously

Resource Allocation Graph


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Resource-Allocation Graph

V is partitioned into two types:

P= {P1, P2, , Pn}, the set consisting of all the processes inthe system

R= {R1, R

2, , R

m}, the set consisting of all resource types

in the system

request edge directed edge PiRj

assignment edge directed edge RjPi

A set of vertices Vand a set of edges E

Resource Allocation Graph (Cont )


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Resource-Allocation Graph (Cont.)

Process

Resource Type with 4 instances

Pirequests instance of Rj

Pi is holding an instance of Rj Pi

Pi

Rj

Rj

Example of a Resource Allocation Graph


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Example of a Resource Allocation Graph

Resource Allocation Graph With A Deadlock


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Resource Allocation Graph With A Deadlock

Graph With A Cycle But No Deadlock


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Graph With A Cycle But No Deadlock

Basic Facts


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Basic Facts

If graph contains no cycles no deadlock

If graph contains a cycle

if only one instance per resource type, then deadlock

if several instances per resource type, possibility of deadlock

Methods for Handling Deadlocks


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Methods for Handling Deadlocks

Ensure that the system will never enter a deadlock state

Allow the system to enter a deadlock state and then recover

Ignore the problem and pretend that deadlocks never occur

in the system; used by most operating systems, includingUNIX

Deadlock Prevention


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Deadlock Prevention

Mutual Exclusion not required for sharableresources; must hold for nonsharable resources

Hold and Wait must guarantee that whenever a

process requests a resource, it does not hold any otherresources

Require process to request and be allocated all itsresources before it begins execution, or allow process torequest resources only when the process has none

Low resource utilization; starvation possible

Restrain the ways request can be made

Deadlock Prevention (Cont.)


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Deadlock Prevention (Cont.)

No Preemption

If a process that is holding some resources requests anotherresource that cannot be immediately allocated to it, then allresources currently being held are released

Preempted resources are added to the list of resources forwhich the process is waiting

Process will be restarted only when it can regain its oldresources, as well as the new ones that it is requesting

Circular Wait impose a total ordering of all resourcetypes, and require that each process requests resources in

an increasing order of enumeration

Deadlock Avoidance


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Deadlock Avoidance

Simplest and most useful model requires that eachprocess declare the maximum numberof resources ofeach type that it may need

The deadlock-avoidance algorithm dynamically examinesthe resource-allocation state to ensure that there cannever be a circular-wait condition

Resource-allocation stateis defined by the number ofavailable and allocated resources, and the maximumdemands of the processes

Requires that the system has some additional a prioriinformationavailable


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Silberschatz, Galvin and Gagne Operating System Concepts Essentials 8th Edition

End of Chapter 6

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Process Synchronization and Deadlock