Lecture#5

Silberschatz, Galvin and Gagne ©2013Operating System Concepts – 9th
Edition
Chapter 5: Process
Synchronization

5.2 Silberschatz, Galvin and Gagne ©2013Operating System Concepts – 9th
Edition
Chapter 6: Process Scheduling
Background
The Critical-Section Problem
Peterson’s Solution
Synchronization Hardware
Mutex Locks
Semaphores
Classic Problems of Synchronization
Monitors
Synchronization Examples
Alternative Approaches

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Objectives
To introduce the critical-section problem, whose solutions can be
used to ensure the consistency of shared data
To present both software and hardware solutions of the critical-
section problem
To examine several classical process-synchronization problems
To explore several tools that are used to solve process
synchronization problems

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Background
Processes can execute concurrently
May be interrupted at any time, partially completing
execution
Concurrent access to shared data may result in data
inconsistency
Maintaining data consistency requires mechanisms to
ensure the orderly execution of cooperating processes

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Interleaving and Overlapping Processes
processes may be interleaved on uniprocessors

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Interleaving and
Overlapping Processes
And not only interleaved but overlapped on
multi-processors

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Producer-Consumer Problem
in counter out

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Producer
while (true) {
/* produce an item in next produced */
while (counter == BUFFER SIZE)
; /* do nothing */
buffer[in] = next produced;
in = (in + 1) % BUFFER SIZE;
counter++;
}

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Consumer
while (true) {
while (counter == 0)
; /* do nothing */
next consumed = buffer[out];
out = (out + 1) % BUFFER SIZE;
counter--;
/* consume the item in next consumed */
}

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Race Condition
A race condition occurs when
Multiple processes or threads read and write data items
They do so in a way where the final result depends on
the order of execution of the processes.
The output depends on who finishes the race last.

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Race Condition: Example
counter++ could be implemented as
register1 = counter
register1 = register1 + 1
counter = register1
counter-- could be implemented as
register2 = counter
register2 = register2 - 1
counter = 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 {counter = 6 }
S5: consumer execute counter = register2 {counter = 4}

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Enforce Single Access
If we enforce a rule that only one process may enter the function at a time
then:
P1 & P2 run on separate processors
P1 enters echo first,
P2 tries to enter but is blocked – P2 suspends
P1 completes execution
P2 resumes and executes echo

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Competition among
Processes for Resources
Three main control problems:
Need for Mutual Exclusion
Critical sections
Deadlock
Starvation

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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 exit section, then remainder
section

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Critical Section
General structure of process pi is

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16
Critical Regions (2)
Mutual exclusion using critical regions

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Solution to Critical-Section Problem
1. Mutual Exclusion - If process Pi is executing in its critical section, then no
other processes can be executing in their critical sections
2. Progress - If no process is executing in its critical section and there exist some
processes that wish to enter their critical section, then the selection of the
processes that will enter the critical section next cannot be postponed
indefinitely
3. Bounded Waiting - A bound must exist on the number of times that other
processes are allowed to enter their critical sections after a process has made
a request to enter its critical section and before that request is granted
 Assume that each process executes at a nonzero speed
 No assumption concerning relative speed of the n processes
Two approaches depending on if kernel is preemptive or non-preemptive
Preemptive – allows preemption of process when running in kernel mode
Non-preemptive – runs until exits kernel mode, blocks, or voluntarily
yields CPU
Essentially free of race conditions in kernel mode

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Mutual Exclusion with Busy Waiting
Proposed solution to critical region problem
(a) Process 0. (b) Process 1.

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Peterson’s Solution
Good algorithmic description of solving the problem
Two process solution
Assume that the load and store instructions are atomic; that is, cannot be
interrupted
The two processes share two variables:
int turn;
Boolean flag[2]
The variable turn indicates whose turn it is to enter the critical section
The flag array is used to indicate if a process is ready to enter the critical
section. flag[i] = true implies that process Pi is ready!

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do {
flag[i] = true;
turn = j;
while (flag[j] && turn == j);
critical section
flag[i] = false;
remainder section
} while (true);
Provable that
1. Mutual exclusion is preserved
2. Progress requirement is satisfied
3. Bounded-waiting requirement is met
Algorithm for Process Pi

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Synchronization Hardware
Many systems provide hardware support for critical section code
All solutions below based on idea of locking
Protecting critical regions via locks
Uniprocessors – could disable interrupts
Currently running code would execute without preemption
Generally too inefficient on multiprocessor systems
 Operating systems using this not broadly scalable
Modern machines provide special atomic hardware instructions
 Atomic = non-interruptible
Either test memory word and set value
Or swap contents of two memory words

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do {
acquire lock
critical section
release lock
remainder section
} while (TRUE);
Solution to Critical-section Problem Using Locks

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23
Test & Set Lock Register
Entering and leaving a critical region using the
TSL instruction

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test_and_set Instruction
Definition:
boolean test_and_set (boolean *target)
{
boolean rv = *target;
*target = TRUE;
return rv:
}

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Solution using test_and_set()
Shared boolean variable lock, initialized to FALSE
Solution:
do {
while (test_and_set(&lock))
; /* do nothing */
/* critical section */
lock = false;
/* remainder section */
} while (true);

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Mutex Locks
Previous solutions are complicated and generally inaccessible to
application programmers
OS designers build software tools to solve critical section problem
Simplest is mutex lock
Product critical regions with it by first acquire() a lock then
release() it
Boolean variable indicating if lock is available or not
Calls to acquire() and release() must be atomic
Usually implemented via hardware atomic instructions
But this solution requires busy waiting
This lock therefore called a spinlock

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acquire() and release()
acquire() {
while (!available)
; /* busy wait */
available = false;;
}
release() {
available = true;
}
do {
acquire lock
critical section
release lock
remainder section
} while (true);

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Semaphore
Synchronization tool that does not require busy waiting
Semaphore S – integer variable
Two standard operations modify S: wait() and signal()
Originally called P() and V()
Less complicated
Can only be accessed via two indivisible (atomic) operations
}

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Semaphore Primitives
DOWN
UP

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Semaphore Usage
Counting semaphore – integer value can range over an unrestricted domain
Binary semaphore – integer value can range only between 0 and 1
Then a mutex lock
Can implement a counting semaphore S as a binary semaphore
Can solve various synchronization problems
Consider P1 and P2 that require S1 to happen before S2
P1:
S1;
signal(synch);
P2:
wait(synch);
S2;

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Semaphore Implementation
Must guarantee that no two processes can execute wait () and signal ()
on the same semaphore at the same time
Thus, implementation becomes the critical section problem where the wait and
signal code are placed in the critical section
Could now have busy waiting in 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

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Binary Semaphore Primitives

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Mutual Exclusion Using Semaphores

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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 of the waiting processes
Let S and Q be two semaphores initialized to 1
P0 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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The producer-consumer problem

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Dining-Philosophers Problem
Philosophers spend their lives thinking and eating
Don’t 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

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39
Dining Philosophers (2)
A non-solution to the dining philosophers problem

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40

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41

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42
The Sleeping Barber Problem (1)

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#define CHAIRS 5 /* # chairs for waiting customers */
typedef int semaphore; /* use your imagination */
semaphore customers = 0; /* # of customers waiting for service */
semaphore barbers = 0; /* # of barbers waiting for customers */
semaphore mutex = 1; /* for mutual exclusion */
int waiting = 0; /* customer are waiting (not being cut) */
void barber(void)
{
while (TRUE) {
down(&customers); /* go to sleep if # of customers is 0 */
down(&mutex); /* acquire access to "waiting' */
waiting = waiting - 1; /* decrement count of waiting customers */
up(&barbers); /* one barber is now read y to cut hair */
up(&mutex); /* release 'waiting' */
cut_hair(); /* cut hair (outside critical region */
}
}
void customer(void)
{
down(&mutex); /* enter critical region */
if (waiting < CH AIRS) { /* if there are no free chairs, leave */
waiting = waiting + 1; /* increment count of waiting customers */
up(&customers); /* wake up barber if necessary */
up(&mutex); /* release access to 'waiting' */
down(&barbers); /* go to sleep if # of free barbers is 0 */
get_haircut(); /* be seated and be served */
} else {
up(&mutex); /* shop is full; do not wait */
}
}

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Monitors
A high-level abstraction that provides a convenient and effective mechanism for
process synchronization
Abstract data type, internal variables only accessible by code within the procedure
Only one process may be active within the monitor at a time
But not powerful enough to model some synchronization schemes
monitor monitor-name
{
// shared variable declarations
procedure P1 (…) { …. }
procedure Pn (…) {……}
Initialization code (…) { … }
}
}

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Schematic view of a Monitor

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Condition Variables
condition x, y;
Two operations on a condition variable:
x.wait () – a process that invokes the operation is suspended until x.signal ()
x.signal () – resumes one of processes (if any) that invoked x.wait ()
 If no x.wait () on the variable, then it has no effect on the variable

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Monitor with Condition Variables

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Condition Variables Choices
If process P invokes x.signal (), with Q in x.wait () state, what should happen next?
If Q is resumed, then P must wait
Options include
Signal and wait – P waits until Q leaves monitor or waits for another condition
Signal and continue – Q waits until P leaves the monitor or waits for another condition
Both have pros and cons – language implementer can decide
Monitors implemented in Concurrent Pascal compromise
 P executing signal immediately leaves the monitor, Q is resumed
Implemented in other languages including Mesa, C#, Java

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Synchronization Examples
Solaris
Windows XP
Linux
Pthreads

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