Ch7: Process Synchronization

Silberschatz, Galvin and Gagne ©20027.1
Chapter 7: Process
Synchronization
Background
The Critical-Section Problem
Synchronization Hardware
Semaphores
Classical Problems of Synchronization
Critical Regions
Monitors
Synchronization in Solaris 2 & Windows 2000

Background
Concurrent access to shared data may result in data
inconsistency.
Maintaining data consistency requires mechanisms to
ensure the orderly execution of cooperating processes.
Shared-memory solution to bounded-butter problem
(Chapter 4) allows at most n – 1 items in buffer at the
same time. A solution, where all N buffers are used is not
simple.
Suppose that we modify the producer-consumer code by
adding a variable counter, initialized to 0 and incremented
each time a new item is added to the buffer

Bounded-Buffer
Shared data
#define BUFFER_SIZE 10
typedef struct {
. . .
} item;
item buffer[BUFFER_SIZE];
int in = 0;
int out = 0;
int counter = 0;

Bounded-Buffer
Producer process
item nextProduced;
while (1) {
while (counter == BUFFER_SIZE)
; /* do nothing */
buffer[in] = nextProduced;
in = (in + 1) % BUFFER_SIZE;
counter++;
}

Bounded-Buffer
Consumer process
item nextConsumed;
while (1) {
while (counter == 0)
; /* do nothing */
nextConsumed = buffer[out];
out = (out + 1) % BUFFER_SIZE;
counter--;
}

Bounded Buffer
The statements
counter++;
counter--;
must be performed atomically.
Atomic operation means an operation that completes in
its entirety without interruption.

Bounded Buffer
The statement “count++” may be implemented in
machine language as:
register1 = counter
register1 = register1 + 1
counter = register1
The statement “count—” may be implemented as:
register2 = counter
register2 = register2 – 1
counter = register2

Bounded Buffer
If both the producer and consumer attempt to update the
buffer concurrently, the assembly language statements
may get interleaved.
Interleaving depends upon how the producer and
consumer processes are scheduled.

Bounded Buffer
Assume counter is initially 5. One interleaving of
statements is:
producer: register1 = counter (register1 = 5)
producer: register1 = register1 + 1 (register1 = 6)
consumer: register2 = counter (register2 = 5)
consumer: register2 = register2 – 1 (register2 = 4)
producer: counter = register1 (counter = 6)
consumer: counter = register2 (counter = 4)
The value of count may be either 4 or 6, where the
correct result should be 5.

Race Condition
Race condition: The situation where several processes
access – and manipulate shared data concurrently. The
final value of the shared data depends upon which
process finishes last.
To prevent race conditions, concurrent processes must
be synchronized.

The Critical-Section Problem
n processes all competing to use some shared data
Each process has a code segment, called critical section,
in which the shared data is accessed.
Problem – ensure that when one process is executing in
its critical section, no other process is allowed to execute
in its critical section.

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.

Initial Attempts to Solve Problem
Only 2 processes, P0 and P1
General structure of process Pi (other process Pj)
do {
entry section
critical section
exit section
reminder section
} while (1);
Processes may share some common variables to
synchronize their actions.

Algorithm 1
Shared variables:
int turn;
initially turn = 0
turn - i ⇒ Pi can enter its critical section
Process Pi
do {
while (turn != i) ;
critical section
turn = j;
reminder section
} while (1);
Satisfies mutual exclusion, but not progress

Algorithm 2
Shared variables
boolean flag[2];
initially flag [0] = flag [1] = false.
flag [i] = true ⇒ Pi ready to enter its critical section
Process Pi
do {
flag[i] := true;
while (flag[j]) ;
critical section
flag [i] = false;
remainder section
} while (1);
Satisfies mutual exclusion, but not progress requirement.

Algorithm 3
Combined shared variables of algorithms 1 and 2.
Process Pi
do {
flag [i]:= true;
turn = j;
while (flag [j] and turn = j) ;
critical section
flag [i] = false;
remainder section
} while (1);
Meets all three requirements; solves the critical-section
problem for two processes.

Bakery Algorithm
Before entering its critical section, process receives a
number. Holder of the smallest number enters the critical
section.
If processes Pi and Pj receive the same number, if i < j,
then Pi is served first; else Pj is served first.
The numbering scheme always generates numbers in
increasing order of enumeration; i.e., 1,2,3,3,3,3,4,5...
Critical section for n processes

Bakery Algorithm
Notation <≡ lexicographical order (ticket #, process id #)
(a,b) < c,d) if a < c or if a = c and b < d
max (a0,…, an-1) is a number, k, such that k ≥ ai for i - 0,
…, n – 1
Shared data
boolean choosing[n];
int number[n];
Data structures are initialized to false and 0 respectively

Bakery Algorithm
do {
choosing[i] = true;
number[i] = max(number[0], number[1], …, number [n –
1])+1;
choosing[i] = false;
for (j = 0; j < n; j++) {
while (choosing[j]) ;
while ((number[j] != 0) && (number[j,j] <
number[i,i])) ;
}
critical section
number[i] = 0;
remainder section
} while (1);

Test and modify the content of a word atomically
.
boolean TestAndSet(boolean &target) {
boolean rv = target;
tqrget = true;
return rv;
}

Mutual Exclusion with Test-and-Set
Shared data:
boolean lock = false;
Process Pi
do {
while (TestAndSet(lock)) ;
critical section
lock = false;
remainder section
}

Atomically swap two variables.
void Swap(boolean &a, boolean &b) {
boolean temp = a;
a = b;
b = temp;
}

Mutual Exclusion with Swap
Shared data (initialized to false):
boolean lock;
boolean waiting[n];
Process Pi
do {
key = true;
while (key == true)
Swap(lock,key);
critical section
lock = false;
remainder section
}

Semaphores
Synchronization tool that does not require busy waiting.
Semaphore S – integer variable
can only be accessed via two indivisible (atomic)
operations
wait (S):
while S≤ 0 do no-op;
S--;
signal (S):
S++;

Critical Section of n Processes
Shared data:
semaphore mutex; //initially mutex = 1
Process Pi:
do {
wait(mutex);
critical section
signal(mutex);
remainder section
} while (1);

Semaphore Implementation
Define a semaphore as a record
typedef struct {
int value;
struct process *L;
} semaphore;
Assume two simple operations:
block suspends the process that invokes it.
wakeup(P) resumes the execution of a blocked process P.

Implementation
Semaphore operations now defined as
wait(S):
S.value--;
if (S.value < 0) {
add this process to S.L;
block;
}
signal(S):
S.value++;
if (S.value <= 0) {
remove a process P from S.L;
wakeup(P);
}

Semaphore as a General Synchronization
Tool
Execute B in Pj only after A executed in Pi
Use semaphore flag initialized to 0
Code:
Pi Pj
 
A wait(flag)
signal(flag) B

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.

Two Types of Semaphores
Counting semaphore – integer value can range over
an unrestricted domain.
Binary semaphore – integer value can range only
between 0 and 1; can be simpler to implement.
Can implement a counting semaphore S as a binary
semaphore.

Implementing S as a Binary
Semaphore
Data structures:
binary-semaphore S1, S2;
int C:
Initialization:
S1 = 1
S2 = 0
C = initial value of semaphore S

Implementing S
wait operation
wait(S1);
C--;
if (C < 0) {
signal(S1);
wait(S2);
}
signal(S1);
signal operation
wait(S1);
C ++;
if (C <= 0)
signal(S2);
else
signal(S1);

Classical Problems of
Synchronization
Bounded-Buffer Problem
Readers and Writers Problem
Dining-Philosophers Problem

Bounded-Buffer Problem
Shared data
semaphore full, empty, mutex;
Initially:
full = 0, empty = n, mutex = 1

Bounded-Buffer Problem Producer
Process
do {
…
produce an item in nextp
…
wait(empty);
wait(mutex);
…
add nextp to buffer
…
signal(mutex);
signal(full);
} while (1);

Bounded-Buffer Problem Consumer
Process
do {
wait(full)
wait(mutex);
…
remove an item from buffer to nextc
…
signal(mutex);
signal(empty);
…
consume the item in nextc
…
} while (1);

Readers-Writers Problem
Shared data
semaphore mutex, wrt;
Initially
mutex = 1, wrt = 1, readcount = 0

Readers-Writers Problem Writer Process
wait(wrt);
…
writing is performed
…
signal(wrt);

Readers-Writers Problem Reader
Process
wait(mutex);
readcount++;
if (readcount == 1)
wait(rt);
signal(mutex);
…
reading is performed
…
wait(mutex);
readcount--;
if (readcount == 0)
signal(wrt);
signal(mutex):

Shared data
semaphore chopstick[5];
Initially all values are 1

Philosopher i:
do {
wait(chopstick[i])
wait(chopstick[(i+1) % 5])
…
eat
…
signal(chopstick[i]);
signal(chopstick[(i+1) % 5]);
…
think
…
} while (1);

Critical Regions
High-level synchronization construct
A shared variable v of type T, is declared as:
v: shared T
Variable v accessed only inside statement
region v when B do S
where B is a boolean expression.
While statement S is being executed, no other process
can access variable v.

Critical Regions
Regions referring to the same shared variable exclude
each other in time.
When a process tries to execute the region statement, the
Boolean expression B is evaluated. If B is true, statement
S is executed. If it is false, the process is delayed until B
becomes true and no other process is in the region
associated with v.

Example – Bounded Buffer
Shared data:
struct buffer {
int pool[n];
int count, in, out;
}

Bounded Buffer Producer Process
Producer process inserts nextp into the shared buffer
region buffer when( count < n) {
pool[in] = nextp;
in:= (in+1) % n;
count++;
}

Bounded Buffer Consumer Process
Consumer process removes an item from the shared
buffer and puts it in nextc
region buffer when (count > 0) {
nextc = pool[out];
out = (out+1) % n;
count--;
}

Implementation region x when B do
S
Associate with the shared variable x, the following
variables:
semaphore mutex, first-delay, second-delay;
int first-count, second-count;
Mutually exclusive access to the critical section is
provided by mutex.
If a process cannot enter the critical section because the
Boolean expression B is false, it initially waits on the
first-delay semaphore; moved to the second-delay
semaphore before it is allowed to reevaluate B.

Implementation
Keep track of the number of processes waiting on first-
delay and second-delay, with first-count and
second-count respectively.
The algorithm assumes a FIFO ordering in the queuing of
processes for a semaphore.
For an arbitrary queuing discipline, a more complicated
implementation is required.

Monitors
High-level synchronization construct that allows the safe sharing
of an abstract data type among concurrent processes.
monitor monitor-name
{
shared variable declarations
procedure body P1 (…) {
. . .
}
procedure body P2 (…) {
. . .
}
procedure body Pn (…) {
. . .
}
{
initialization code
}
}

Monitors
To allow a process to wait within the monitor, a
condition variable must be declared, as
condition x, y;
Condition variable can only be used with the
operations wait and signal.
The operation
x.wait();
means that the process invoking this operation is
suspended until another process invokes
x.signal();
The x.signal operation resumes exactly one suspended
process. If no process is suspended, then the signal
operation has no effect.

Schematic View of a Monitor

Monitor With Condition Variables

Dining Philosophers Example
monitor dp
{
enum {thinking, hungry, eating} state[5];
condition self[5];
void pickup(int i) // following
slides
void putdown(int i) // following slides
void test(int i) // following slides
void init() {
for (int i = 0; i < 5; i++)
state[i] = thinking;
}
}

Dining Philosophers
void pickup(int i) {
state[i] = hungry;
test[i];
if (state[i] != eating)
self[i].wait();
}
void putdown(int i) {
state[i] = thinking;
// test left and right neighbors
test((i+4) % 5);
test((i+1) % 5);
}

Dining Philosophers
void test(int i) {
if ( (state[(I + 4) % 5] != eating) &&
(state[i] == hungry) &&
(state[(i + 1) % 5] != eating)) {
state[i] = eating;
self[i].signal();
}
}

Monitor Implementation Using
Semaphores
Variables
semaphore mutex; // (initially = 1)
semaphore next; // (initially = 0)
int next-count = 0;
Each external procedure F will be replaced by
wait(mutex);
…
body of F;
…
if (next-count > 0)
signal(next)
else
signal(mutex);
Mutual exclusion within a monitor is ensured.

Monitor Implementation
For each condition variable x, we have:
semaphore x-sem; // (initially = 0)
int x-count = 0;
The operation x.wait can be implemented as:
x-count++;
if (next-count > 0)
signal(next);
else
signal(mutex);
wait(x-sem);
x-count--;

The operation x.signal can be implemented as:
if (x-count > 0) {
next-count++;
signal(x-sem);
wait(next);
next-count--;
}

Conditional-wait construct: x.wait(c);
c – integer expression evaluated when the wait operation is
executed.
value of c (a priority number) stored with the name of the
process that is suspended.
when x.signal is executed, process with smallest
associated priority number is resumed next.
Check two conditions to establish correctness of system:
User processes must always make their calls on the monitor
in a correct sequence.
Must ensure that an uncooperative process does not ignore
the mutual-exclusion gateway provided by the monitor, and
try to access the shared resource directly, without using the
access protocols.

Solaris 2 Synchronization
Implements a variety of locks to support multitasking,
multithreading (including real-time threads), and
multiprocessing.
Uses adaptive mutexes for efficiency when protecting
data from short code segments.
Uses condition variables and readers-writers locks when
longer sections of code need access to data.
Uses turnstiles to order the list of threads waiting to
acquire either an adaptive mutex or reader-writer lock.

Windows 2000 Synchronization
Uses interrupt masks to protect access to global
resources on uniprocessor systems.
Uses spinlocks on multiprocessor systems.
Also provides dispatcher objects which may act as wither
mutexes and semaphores.
Dispatcher objects may also provide events. An event
acts much like a condition variable.

Ch7: Process Synchronization

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Ch7: Process Synchronization