Lecture 05
Process Synchronization
Background
The Critical-Section Problem
Autumn 2000
M.B. Ibáñez
Outline
• What is the problem with cooperating
processes that share data?
• Mutual Exclusion: Software Approaches
Example taken from:
William Stalling.
Operating Systems. Internals and Design Principles.
Third Edition. Prentice Hall. 1997.
Autumn 2000
M.B. Ibáñez
Example: Counting Contest
static int i
Process A
i=0
while ( i < 10 ) {
i++;
}
System.out.println
(“A wins”);
Autumn 2000
Process B
i=0
while ( i > -10 ) {
i--;
}
System.out.println
(“B wins”);
M.B. Ibáñez
Protocol Igloo
Autumn 2000
M.B. Ibáñez
Concepts
• Mutual Exclusion
– Mechanisms that ensure that only one person or process
is doing certain things at one time
• Critical Section
– A section of code in which only one process may be
executing at a given time
• Synchronization
– The use of atomic operations to ensure the correct
operation of cooperating processes
Autumn 2000
M.B. Ibáñez
First Solution: Busy Waiting
var turn: 0..1
Process 0
…
while turn <> 0 do
{nothing};
< critical section >
turn := 1;
...
Autumn 2000
Process 1
…
while turn <> 1 do
{nothing};
< critical section >
turn := 0;
...
M.B. Ibáñez
Comments
• This solution guarantees the mutual
exclusion property.
• Drawbacks:
– Processes must strictly alternate in their use of their
critical section
– If one process fails, the other process is permanently
blocked
– Busy waiting: the thwarted process can do nothing
productive until it gets permission to enter its critical
section
Autumn 2000
M.B. Ibáñez
Second Solution
var flag: array [0..1] of boolean;
Process 0
…
while flag[1] do
{nothing};
flag[0] := true;
< critical section >;
flag[0] := false;
...
Autumn 2000
Process 1
…
while flag[0] do
{nothing};
flag[1] := true;
< critical section >;
flag[1] := false;
...
M.B. Ibáñez
Comments
• This solution does not guarantee mutual
exclusion. Consider the following sequence:
P0 executes the while statement and finds flag[1] set
to false.
P1 executes the while statement and finds flag[0] set
to false
P0 sets flag[0] to true and enter its critical section
P1 sets flag[1] to true and enter its critical section
Autumn 2000
M.B. Ibáñez
Third Solution
var flag: array [0..1] of boolean;
Process 0
…
flag[0] := true;
while flag[1] do
{nothing};
< critical section >;
flag[0] := false;
...
Autumn 2000
Process 0
…
flag[1] := true;
while flag[0] do
{nothing};
< critical section >;
flag[1] := false;
...
M.B. Ibáñez
Comments
• Mutual exclusion is guarantee
• Drawbacks:
– If one process fails inside its critical section, the
other process is blocked
– If both processes set their flags to true before
either has executed the while statement, then
each think that the other has entered its critical
section, causing deadlock.
Autumn 2000
M.B. Ibáñez
Fourth Solution
Process 0
Process 1
…
flag[0] := true;
while flag[1] do {
flag[0] := false;
<delay for a short time>;
flag[0] := true;
};
<critical section>;
flag[0] := false;
...
…
flag[1] := true;
while flag[0] do {
flag[1] := false;
<delay for a short time>;
flag[1] := true;
};
<critical section>;
flag[1] := false;
...
Autumn 2000
M.B. Ibáñez
Comments
• The mutual exclusion is guaranteed
• Consider the following sequence of events:
P0 sets flag[0] to true
P1 sets flag[1] to true
P0 checks flag[1]
P1 checks flag[0]
P0 sets flag[0] to false
P1 sets flag[1] to false
P0 sets flag[0] to true
P1 sets flag[1] to true
Autumn 2000
M.B. Ibáñez
A Correct Solution
Dekker’s Algorithm I
procedure P0;
begin
repeat
flag[0] := true;
while flag[1] do if turn = 1 then
begin
flag[0] := false;
while turn = 1 do {nothing};
flag[0] := true;
end
<critical section>;
turn := 1;
flag[0] := false;
<remainder>
forever
end;
Autumn 2000
procedure P1;
begin
repeat
flag[1] := true;
while flag[0] do if turn = 0 then
begin
flag[1] := false;
while turn = 0 do {nothing};
flag[1] := true;
end
<critical section>;
turn := 0;
flag[1] := false;
<remainder>
forever
end;
M.B. Ibáñez
A Correct Solution
Dekker’s Algorithm II
var flag
array[0..1] of boolean;
turn: 0..1;
Autumn 2000
begin
flag[0] := false;
flag[1] := false;
turn := 1;
parbegin
P0; P1;
parend
end
M.B. Ibáñez