Which condition is necessary to prevent the race condition described?

Question

BlackTom AI · Accepted Answer

Mutual exclusion (allowing only one actor at a time to access a shared resource) is the core requirement to prevent the described race condition.

1) Relevant concepts
- Race condition: occurs when two or more processes/threads access and modify shared data concurrently and the final result depends on the timing (interleaving) of their operations.
- Critical section: the code region that reads/modifies shared state (the counter).
- Mutual exclusion/synchronization: mechanisms (locks, semaphores, monitors, atomic instructions) that ensure only one process executes the critical section at a time.

2) Step-by-step reasoning (interleaving example)
Consider two processes A and B both incrementing a shared counter initialized to 5. Increment typically breaks into three sub-steps: Read (R), Compute (C: add 1), and Write (W).
- A:R reads 5
- A:C computes 6
- (Context switch happens before A:W)
- B:R reads 5
- B:C computes 6
- B:W writes 6
- A:W writes 6
Final counter = 6, but two increments should have produced 7. This is a classic lost-update caused by concurrent access without mutual exclusion.

3) Intermediate logical steps shown above
- Read values: both read 5
- Compute increments: both compute 6 independently
- Writes: last write wins, causing one increment to be lost

4) Why the correct answer is correct
Ensuring only one process at a time can manipulate the counter (mutual exclusion) prevents the interleaving that caused the lost update. If a lock or atomic operation is used, the sequence for one process completes (R, C, W) before another process can enter the critical section, guaranteeing correct cumulative updates.

5) Why the other options are incorrect
- Using virtual memory instead of physical memory: memory abstraction does not address concurrent access ordering; it’s unrelated to synchronization.
- Running processes on separate CPUs: separate CPUs can still access the same shared memory concurrently and cause the same race unless synchronization is used.
- Increasing execution speed of processes: speed does not remove interleavings and can make timing even less predictable; it does not guarantee atomicity.

6) Conclusion
Mutual exclusion—ensuring only one process manipulates the counter at a time—is necessary to prevent the race condition. This matches the provided correct answer.

Which condition is necessary to prevent the race condition described?单项选择题

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