Operating Systems
Synchronisation
Stephan Sigg
Distributed and Ubiquitous Systems
Technische Universität Braunschweig
February 10, 2011
Stephan Sigg Operating Systems 1/79
Overview and Structure
Introduction to operating systems
History
Architectures
Processes
Processes, Threads, IPC, Scheduling
Synchronisation
Deadlocks
Memory management
Paging
Segmentation
Filesystems
Security and Protection
Distributed systems
Cryptography
Stephan Sigg Operating Systems 2/79
Outline
Synchronisation
1 Synchronisation
The critical section problem
Synchronisation hardware
Semaphores
Classic synchronisation problems
Monitors
Atomic transactions
Stephan Sigg Operating Systems 3/79
Synchronisation
Introduction
Definition
A cooperating process is one that can affect or be affected by
other processes executing in the system
Cooperating processes can either directly share a logical
address space (code and data)
Or share data through files or messages
Concurrent access to shared data may result in data
inconsistency
Stephan Sigg Operating Systems 4/79
Synchronisation
Introduction
The producer-consumer-problem
Example
T0 : producer execute register1 = counter {register1 = 5}
T1 : producer execute register1 = register1 + 1 {register1 = 6}
T2 : consumer execute register2 = counter {register2 = 5}
T3 : consumer execute register2 = register2 − 1 {register2 = 4}
T4 : producer execute counter = register1 {counter = 6}
T5 : consumer execute counter = register2 {counter = 4}
Stephan Sigg Operating Systems 5/79
Synchronisation
Introduction
The producer-consumer-problem
Definition
A situation where processes access and manipulate the same data
concurrently and the outcome of the execution depends on the
order in which access takes place is called a race condition
To guard against race condition we need to ensure that only
one process at a time can be manipulating the shared data.
Synchronisation among processes is required
Stephan Sigg Operating Systems 6/79
Outline
Synchronisation
1 Synchronisation
The critical section problem
Synchronisation hardware
Semaphores
Classic synchronisation problems
Monitors
Atomic transactions
Stephan Sigg Operating Systems 7/79
Synchronisation
The critical section problem
The critical section problem
Consider a system of n processes {P0,P1, . . . ,Pn−1}
Each process has a segment of code called its critical section
In the critical section, processes change common variables,
update a table, write a file, . . .
Only one process is allowed to execute its critical section at a
time
The critical section problem is to design a protocol that
enables this property
Stephan Sigg Operating Systems 8/79
Synchronisation
The critical section problem
The critical section problem
Structure of a typical process Pi :
1: while true do
2: entry section
3: critical section
4: exit section
5: remainder section
6: end while
remainder
section
critical
section
exit
section
entry
section
remainder
section
critical
section
exit
section
entry
section
remainder
section
critical
section
exit
section
entry
section
remainder
section
critical
section
exit
section
entry
section
Stephan Sigg Operating Systems 9/79
Synchronisation
The critical section problem
A solution to the critical section
problem must satisfy the following
requirements:
Mutual exclusion
Progress
Bounded waiting
remainder
section
critical
section
exit
section
entry
section
remainder
section
critical
section
exit
section
entry
section
remainder
section
critical
section
exit
section
entry
section
remainder
section
critical
section
exit
section
entry
section
Stephan Sigg Operating Systems 10/79
Synchronisation
The critical section problem
Mutual exclusion
If process Pi is executed in its
critical section, no other
processes are allowed to be
executed in their critical sections
remainder
section
critical
section
exit
section
entry
section
remainder
section
critical
section
exit
section
entry
section
remainder
section
critical
section
exit
section
entry
section
remainder
section
critical
section
exit
section
entry
section
Stephan Sigg Operating Systems 11/79
Synchronisation
The critical section problem
Progress
If no process is executing in its
critical section
Only those processes that are
not executing in their remainder
section participate in deciding
wether to enter the critical
section
remainder
section
critical
section
exit
section
entry
section
remainder
section
critical
section
exit
section
entry
section
remainder
section
critical
section
exit
section
entry
section
remainder
section
critical
section
exit
section
entry
section
Stephan Sigg Operating Systems 12/79
Synchronisation
The critical section problem
Bounded waiting
It exists a bound on the number
of times that other processes are
allowed to enter their critical
section after a request to enter
the critical section of a specific
process is answered.
remainder
section
critical
section
exit
section
entry
section
remainder
section
critical
section
exit
section
entry
section
remainder
section
critical
section
exit
section
entry
section
remainder
section
critical
section
exit
section
entry
section
Stephan Sigg Operating Systems 13/79
Synchronisation
The critical section problem
Peterson’s solution
A software solution
Synchronisation of two processes Pi
and Pj
The processes share the two data
items
int turn;
boolean flag[2];
Process A
Kernel
Process B
turn
flag[2]
Stephan Sigg Operating Systems 14/79
Synchronisation
The critical section problem
Peterson’s solution
Structure of process Pi
1: while (true) do
2: flag [i ] = true;
3: turn = j ;
4: while (flag [j ]&&turn == j) do
5: nothing
6: end while
7: critical section
8: flag [i ] = false;
9: remainder section
10: end while
Process A
Kernel
Process B
turn
flag[2]
Stephan Sigg Operating Systems 15/79
Synchronisation
The critical section problem
Peterson’s solution
1 This implementation preserves
1 Mutual exclusion
2 Progress requirement
3 Bounded-waiting requirement
2 Software-based solutions are not
guaranteed to work on modern
computer architectures
Modern CPUs reorder memory
accesses to improve execution
efficiency
3 Any solution to solve the critical
section problem requires a lock
Process A
Kernel
Process B
turn
flag[2]
Stephan Sigg Operating Systems 16/79
Outline
Synchronisation
1 Synchronisation
The critical section problem
Synchronisation hardware
Semaphores
Classic synchronisation problems
Monitors
Atomic transactions
Stephan Sigg Operating Systems 17/79
Synchronisation
Synchronisation hardware
Synchronisation hardware
Since software-based solutions are not guaranteed to work on
modern computer architectures, locks are utilised
Race conditions are prevented as critical sections are
protected by locks
Example: lock
1: while true do
2: acquire lock
3: critical section
4: release lock
5: remainder section
6: end while
Stephan Sigg Operating Systems 18/79
Synchronisation
Synchronisation hardware
Synchronisation hardware
Many modern computer systems
provide special hardware instructions
to test and modify the content of a
word
to swap contents of two words as
one uninterruptable unit
We can use these instructions to solve
the critical section problem
Stephan Sigg Operating Systems 19/79
Synchronisation
Synchronisation hardware
TestAndSet()
1: boolean TestAndSet(boolean target){
2: boolean rv = target;
3: target = true;
4: return rv;
5: }
Swap()
1: void Swap(boolean a, boolean b){
2: boolean temp = a;
3: a = b;
4: b = temp;
5: }
Stephan Sigg Operating Systems 20/79
Synchronisation
Synchronisation hardware
Mutual exclusion with TestAndSet()
1: while true do
2: while TestAndSet(&lock) do
3: ; // do nothing
4: end while
5: // critical section
6: lock = false;
7: // remainder section
8: end while
Stephan Sigg Operating Systems 21/79
Synchronisation
Synchronisation hardware
Mutual exclusion with Swap()
1: while true do
2: key = true;
3: while key do
4: Swap(&lock, &key);
5: end while
6: // critic
