ITEC 502 컴퓨터 시스템 및 실습
Chapter 3-1:
Process Synchronization
Mi-Jung Choi
[email protected]
DPNM Lab. Dept. of CSE, POSTECH
Contents
1.
2.
3.
4.
5.
6.
Race Conditions
Critical Regions
Mutual Exclusion with Busy Waiting
Sleep and Wakeup
Semaphores
Mutexes
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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 cooperating processes
 Suppose that we wanted to provide a solution to the
consumer-producer problem that fills all the buffers
– We can do so by having an integer counter that keeps track of
the number of items in the buffer
– Initially, counter is set to 0.
– It is incremented by the producer after it produces a new item
– It is decremented by the consumer after it consumes a item
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Interprocess Communication
 IPC for short
 Race conditions
– may occur in a system
where multiple
processes work
together
– situations where the
final outcome depends
on who runs precisely
when
– Example: spool system
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Race Condition (1)
 Although the producer and consumer routines are correct
separately, they may not function correctly when
executed concurrently
 Suppose
– that the value of the variable counter is currently 5 and
– that the producer and consumer execute the statements
“counter++” and “counter--” concurrently
 Following the execution of these two statements,
the value of the variable counter may be 4, 5, or 6
 Why?
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Race Condition (2)
 We would arrive at this incorrect state
– Because we allowed both processes to manipulate the variable
counter concurrently
 Race Condition
– Several processes access and manipulate the same data
concurrently and the outcome of the execution depends on the
particular order in which the access takes place
 To prevent the race condition
– We need to ensure that only one process at a time can manipulate
the variable counter
– We require that the processes be synchronized in some way
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Critical-Region (CR) Problem
 A system consisting of n processes {P0, P1, …, Pn-1}
– Each process has a segment of code, called a critical region,
where the process may change common variables, updating a
table, writing a file, and so forth
– The system requires that no two processes are executing in their
critical region at the same time
 Solution to the critical-region problem
– To design a protocol that the processes can use to cooperate
– Each process must request permission to enter its CR
– The section of code implementing this request is the entry
section
– The CR may be followed by an exit section
– The remaining code is the remainder section
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Critical-Region Problem
 The general structure of a typical process Pi
Do
{
entry section
critical region
exit section
remainder section
} while ( TRUE );
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Critical Regions (1)
 Mutual exclusion to avoid race conditions
 Four conditions to provide mutual exclusion
–
–
–
–
C1:
C2:
C3:
C4:
No
No
No
No
two processes simultaneously in CR
assumptions about speeds or numbers of CPUs
process outside its CR may block another process
process should have to wait forever to enter its CR
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Critical Regions (2)
Mutual exclusion using critical regions
- This is what we want
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Solution to Critical-Region Problem
 A solution to the critical-region problem must satisfy the
following three requirements:
1. (c1) Mutual Exclusion - If process Pi is executing in its critical
region, then no other processes can be executing in their critical
regions
2. (c3) Progress - If no process is executing in its critical region and
there exist some processes that wish to enter their critical region,
then the selection of the processes that will enter the critical region
next cannot be postponed indefinitely – deadlock-free condition
3. (c4) Bounded Waiting - A bound must exist on the number of
times that other processes are allowed to enter their critical regions
after a process has made a request to enter its critical region and
before that request is granted – starvation-free condition
 (c2) Assume that each process executes at a nonzero speed
 (c2) No assumption concerning relative speed of the n processes
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Peterson’s Solution for critical region problem
 Two process {Pi, Pj} solution
– Software-based 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 region
– The flag array is used to indicate if a process is
ready to enter the critical region
– flag[i] = true implies that process Pi is ready!
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Algorithm for Process Pi
do
{
flag[i] = TRUE;
turn = j;
while ( flag[j] && turn == j);
// entry section
CRITICAL Region
flag[i] = FALSE;
// exit section
REMAINDER Section
} while (TRUE);
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Peterson’s Solution satisfies 3 conditions
 Mutual Exclusion
– Each Pi enters its critical region only if either
flag[j]==false or turn==i
– Each Pi enters its critical region with flag[i]==true
– Both Pi and Pj cannot enter their critical region at the
same time
 Progress
– Pi can be stuck only if either flag[j]==true and
turn==j
 Bounded Waiting
– Pi will enter the critical region after at most one entry
by Pj
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Mutual Exclusion with Busy Waiting (1)
 Disabling interrupts
– disable interrupts while a process in CR
– what happens if there is another CPU??  violates C2!
 Lock variables
– shared (lock) variable, initially 0
– set it to 1 to enter and to 0 to leave CR
– do not solve the problem at all!
 Strict alternation
– processes A and B enter CR alternatively
– violates C3
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Mutual Exclusion with Busy Waiting (2)
Peterson's solution for achieving mutual exclusion
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Mutual Exclusion with Busy Waiting (3)
TSL RX, LOCK
(1) copy value at LOCK to RX
(2) store non-zero at LOCK
* these two steps are indivisible!
Entering and leaving a critical region using the TSL instruction
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Sleep and Wakeup (1)
 Both Peterson’s and TSL solutions are correct, but…
– they require busy waiting  waste CPU cycles
– also, the priority inversion problem may occur!
 Blocking sys. calls preferable to busy-waiting
 Sleep()
– block itself
 Wakeup(pid)
– unblock another process ‘pid’
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Sleep and Wakeup (2)
Producer-consumer problem with fatal race condition
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Semaphore (1)
 The various hardware-based solutions to the critical
region problem
– complicated for application programmers to use
 To overcome this difficulty
– Semaphore may be used
 Semaphore is …
– Synchronization tool that does not require busy waiting
– Semaphore S – integer variable
– Two standard operations to modify S: down() and up()
– The modification of the semaphore is atomic
• When one process modifies the semaphore value, no other process
can simultaneously modify that same semaphore value
– less complicated to use
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Semaphore (2)
 Semaphore
Alphabet
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Semaphores (3)
 Problem with sleep/wakeup
– wakeup signal can be lost  wakeup waiting bit
– what if there are many processes involved??
 we need a counter for wakeups!
 motivation of semaphores
 Semaphores
– integer variables with atomic operations down() and up()
– down(s):
• If (s == 0) then sleep()
• else s = s-1
– up(s):
• s = s+1
• if there is another process sleeping on ‘s’, wake it up
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Usage of Semaphore (1)

Binary semaphore
– Integer value can range only between 0 and 1
– Which is refer to as mutex locks

Binary semaphore for solving the
critical-region problem for
multiple processes
– n processes share a semaphore, mutex
– mutex is initialized to 1
– The structure of a process Pi
do
{
down (mutex);
CRITICAL Region
up (mutex);
REMAINDER Section
} while (TRUE);
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Usage of Semaphore (2) - Binary Semaphore
 Does previous code satisfy three requirements of
CR problem
– Mutual exclusion
– Progress
– Bounded waiting
 The third requirement is not guaranteed as a
default
– It usually depends on the implementation of down()
function
– Ex, Linux sem_wait() does not guarantee the bounded
waiting request
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Semaphores (4)
The producer-consumer problem using semaphores
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Semaphores (5)
void producer(void)
void consumer(void)
#define N 100
{
typedef int semaphore; {
int item;
int item;
semaphore mutex = 1;
semaphore empty = N;
while (TRUE) {
while (TRUE) {
semaphore full = 0;
item = produce_item();
down(&full);
down(&empty);
down(&mutex);
down(&mutex);
remove_item(item);
insert_item(item);
up(&mutex);
up(&mutex);
up(&empty);
up(&full);
consume_item(item);
}
}
}
}
The producer-consumer problem using semaphores
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Mutexes
Implementation of mutex_lock and mutex_unlock
 Can be easily implemented in user space with the help of
TSL
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Summary
 Cooperating processes that share data have critical region
of code each of which is in use by only one process at a
time
 Three requirements for critical region problem
– Mutual Exclusion, Progress, Bounded Waiting
 The main disadvantage of these solution is that they all
require busy waiting. Semaphore overcome this difficulty
 In the next class:
– Semaphore can be used to solve various synchronization problems
• The bounded-buffer, The readers-writers, The dining-philosophers
problem problems
• Those are examples of a large class of concurrency-control problems
• The solution should be deadlock-free ad starvation-free
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Review
1.
2.
3.
4.
5.
6.
Race Conditions
Critical Regions
Mutual Exclusion with Busy Waiting
Sleep and Wakeup
Semaphores
Mutexes
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