Page 118 - DCAP103_Principle of operating system

Unit 4: Process Management-III

Notes
Figure 4.2: Critical Section B

Wait S
Enter

Critical
Section

Exit
Signal S

The key to preventing trouble involving shared storage is find some way to prohibit more than
one process from reading and writing the shared data simultaneously. That part of the program
where the shared memory is accessed is called the Critical Section. To avoid race conditions and
flawed results, one must identify codes in Critical Sections in each thread. The characteristic
properties of the code that form a Critical Section are:
• Codes that reference one or more variables in a “read-update-write” fashion while any of
those variables is possibly being altered by another thread.

• Codes that alter one or more variables that are possibly being referenced in “read-updata-
write” fashion by another thread.

• Codes use a data structure while any part of it is possibly being altered by another thread.

• 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.

Part of the program where the shared memory is accessed is called the
Critical Section.

4.2 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. Structure of process Pi repeat entry section critical section exit section
remainder section until false.

4.3 Way to Handle 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.

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