OPERATING SYSTEMS
DESIGN AND IMPLEMENTATION
Third Edition
ANDREW S. TANENBAUM
ALBERT S. WOODHULL
Chapter 2
Processes
(part 2)
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved. 0-13-142938-8
How To Prove Mutual Exclusion
•
Mutual Exclusion Properties:
–
–
–
–
No deadlock: the two processes cannot enter
a state where they simultaneously wait for
each other
No livelock: if the two processes execute,
one of them at least progresses to the critical
section
Fairness/Absence of starvation: if one
process requests the privilege to enter the
critical section, it eventually enters the
critical section
Correctness: two processes cannot enter the
critical section simultaneously
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved. 0-13-142938-8
1
Finite Automata
A Simple Automaton
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved. 0-13-142938-8
Synchronized Product
A=BxC
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved. 0-13-142938-8
2
Synchronized Product
Set of Synchronization: all actions wearing the same label
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved. 0-13-142938-8
Automata to Model Interleaving
Executions (pseudo-parallelism)
int B;
1: B = 0;
2: while(!B) {
3:
B = 1;
4:
proc();
5:
B = 0;
6: }
Properties:
- proc() is called infinitely often
- when proc() is called, B = 1.
Assumption:
- proc() does not modify B.
7: exit()
Example of program and property to check
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved. 0-13-142938-8
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Model Through Automaton
Model of a boolean variable
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved. 0-13-142938-8
Modelization Through Automaton
Model of the Simple Program
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved. 0-13-142938-8
4
Modelization Through Automaton
Model of the system
(synchronized product of program + variables)
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved. 0-13-142938-8
Proof of TSL (2 processes)
See start of this on whiteboard
Complete it as an exercise
Synchronized Product of a lock + two processes
with Test and Set Lock
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved. 0-13-142938-8
5
The Producer-Consumer Problem (1)
...
Figure 2-13. The producer-consumer problem
with a fatal race condition.
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved. 0-13-142938-8
The Producer-Consumer Problem (2)
...
Figure 2-13. The producer-consumer problem
with a fatal race condition
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved. 0-13-142938-8
6
Semaphores
sem_t s
-> delcares a semaphore
down(s)
-> decrease the value. If value already 0,
block until value > 0.
up(s)
-> increase the value.
sem_init(s, i) -> initialize the value to i
(sem_get(s)) -> optional, gives the current value
Routines of Semaphores
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved. 0-13-142938-8
The Producer-Consumer Problem (3)
...
Figure 2-14. The producer-consumer problem
using semaphores.
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved. 0-13-142938-8
7
The Producer-Consumer Problem (4)
...
Figure 2-14. The producer-consumer problem
using semaphores.
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved. 0-13-142938-8
Monitors (1)
Figure 2-15. A monitor.
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved. 0-13-142938-8
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Monitors (2)
Figure 2-16. An outline of the
producer-consumer problem
with monitors. Only one
monitor procedure at a
time is active. The buffer
has N slots
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved. 0-13-142938-8
Monitors (3)
Figure 2-16. An outline of the producer-consumer problem with
monitors. Only one monitor procedure at a time is active.
The buffer has N slots
Tanenbaum & Woodhull, Operating Systems: Design and Implementation, (c) 2006 Prentice-Hall, Inc. All rights reserved. 0-13-142938-8
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