Page 106 - DCAP403_Operating System

Unit 6: Process Synchronization

2. Codes that alter one or more variables that are possibly being referenced in “read-updata- Notes
write” fashion by another thread.
3. Codes use a data structure while any part of it is possibly being altered by another thread.
4. Codes alter any part of a data structure while it is possibly in use by another thread.

Here, the important point is that when one process is executing shared modifiable data in its
critical section, no other process is to be allowed to execute in its critical section. Thus, the
execution of critical sections by the processes is mutually exclusive in time.

Figure 6.3: Critical Section
Wait S
enter

Critical Section

exit
Signal S

A way of making sure that if one process is using a shared modifiable data, the other processes

will be excluded from doing the same thing.
Formally, while one process executes the shared variable, all other processes desiring to do so

at the same time moment should be kept waiting; when that process has finished executing the

shared variable, one of the processes waiting; while that process has finished executing the shared
variable, one of the processes waiting to do so should be allowed to proceed. In this fashion, each
process executing the shared data (variables) excludes all others from doing so simultaneously.
This is called Mutual Exclusion.

Note Mutual exclusion needs to be enforced only when processes access shared

modifiable data - when processes are performing operations that do not conflict with one
another they should be allowed to proceed concurrently.
6.2.1 Mutual Exclusion Conditions

If you could arrange matters such that no two processes were ever in their critical sections
simultaneously, you could avoid race conditions. You need four conditions to hold to have a
good solution for the critical section problem (mutual exclusion).

1. No two processes may at the same moment inside their critical sections.
2. No assumptions are made about relative speeds of processes or number of CPUs.
3. No process outside its critical section should block other processes.
4. No process should wait arbitrary long to enter its critical section.

6.2.2 Proposals for Achieving Mutual Exclusion

The mutual exclusion problem is to devise a pre-protocol (or entry protocol) and a post-protocol
(or exist protocol) to keep two or more threads from being in their critical sections at the same
time.

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