Peterson's solution is a classical algorithm that uses shared variables to coordinate access to a critical section for two processes. Each process uses a flag variable to indicate whether it wants to enter the critical section, and a turn variable to designate which process can enter next. A process sets its own flag, then checks the other's flag and turn to determine when it can safely enter the critical section without violating mutual exclusion. While this ensures only one process is in the critical section at a time, it may not work correctly on modern computer architectures due to assumptions about atomic read-write operations. Overall it provides a good conceptual model for solving the mutual exclusion problem in a way that guarantees progress and bounds waiting time.

1. Peterson’s Solution
Presented by Mustahsan Mehmood

2. Peterson’s Solution
A classical software based solution
May not work correctly on modern computer architecture
However, it provides a good algorithmic description
Address the requirements of Mutual Exclusion, Progress and Bounded Wait
❑ Restricted to Two Processes: Pi
and Pj

3. Shared Data Items
int turn
•• Indicates whose turn it is to
enter the critical section
boolean flag[2]
•• Indicate if a process is ready
to enter it’s critical section

4. Structure of process Pi
flag[ i ] = false;
flag [ i ] = true; // Ready to enter it’s critical section
turn = j; // Asking other process if it wants to enter it’s C.S
while ( flag[ j ] && turn == j );
do {
} while(true);
Critical Section
Remainder Section

5. Process Pi
Process Pj
flag[ i ] = false;
flag[ i ] = true;
turn = j;
while ( flag[ j ] && turn == j );
do {
} while(true);
Critical Section
Remainder Section
flag[ j ] = false;
flag[ j ] = true;
turn = i;
while ( flag[ i ] && turn == i );
do {
} while(true);
Critical Section
Remainder Section

6. Solution must satisfy following three requirements:
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 some
processes wish to enter their critical sections, then only those
processes that are not executing in their remainder sections can
participate in deciding which will enter its critical section next, and this
selection cannot be postponed indefinitely
3. Bounded Waiting: There exists a bound, or limit, on the number of
times that other processes are allowed to enter their critical sections
after a process has made a request to enter its critical section and
before that request is granted.

7. THANK YOU