Critical-Section Problem (CS130 notes)

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Critical-Section Problem

What is critical section?

A critical section is a sequence of code that atomically accesses shared state.

Solutions - Three Requirements

• Mutal exclusion

At most one thread holds the lock.

• Progress

If no thread holds the lock and any thread attempts to acquire the lock, then eventually some thread succeeds in acquiring the lock.

• Bounded waiting

If thread T attempts to acquire a lock, then there exists a bound on the number of times other threads can successfully acquire the lock before T does.

Critical-Section Handling in OS

preeptive & non-preeptive

• Preemptive

Allows preemption of process when running in kernel mode.

• Non-preemptive

Runs until exits kernel mode, blocks, or voluntarily yields CPU.
Essentially free of race conditions in kernel mode

Peterson Algorithm

Suitable for two processes. Let’s say $p_{i}$ and $p_{j}$.

int turn;
boolean flag[2];
/* Peterson Algorithm for Pi */
do {
	flag[i] = True;
	turn = j;
	while (flag[j] && turn == j);
	/* critical section */
	flag[i] = False;
	/* remainder section */
} while (True)