b) Describe three scenarios that can lead to race conditions and discuss three techniques to prevent them. (15 marks)

Question

BlackTom AI · Accepted Answer

The prompt asks for three scenarios that can lead to race conditions and three techniques to prevent them, and it specifies 15 marks.

Scenario analysis:
1) Shared Variable Update: When two or more threads attempt to increment the same counter variable concurrently, each thread might read the old value, compute a new value, and write it back. If both read before either writes, one update can be lost, causing the final counter to be lower than expected. This is a classic race condition due to unsynchronized access to a shared memory location. 
2) File Access: When multiple processes try to write to the same file at the same time, writes can interleave, overwrite each other, or result in a corrupted file or partial writes. This occurs because file I/O is a shared resource without proper synchronization, leading to a race between write operations. 
3) Bank Transfers: In a financial system, two transactions might concurrently withdraw from and deposit to the same account. Without proper synchronization, the balance computation can race, leading to an incorrect final balance due to interleaved operations on the account state. This is a timing hazard where critical sections touching the account balance are not atomic with respect to other transactions.

Prevention techniques:
1) Locks and Mutexes: By acquiring a lock around a critical section, only one thread can execute that section at a time, ensuring mutual exclusion. For example, a mutex protects increments to a shared counter so that the read-modify-write sequence is atomic with respect to other threads. This directly addresses race conditions caused by concurrent access. 
2) Semaphores: Semaphores can regulate access to a finite set of resources or control concurrent access. A binary semaphore can enforce mutual exclusion similar to a mutex, while counting semaphores can limit the number of threads performing a certain operation, preventing overlapping access that could cause races. 
3) Atomic Operations: Using atomic primitives (e.g., atomic increment, compare-and-swap) ensures certain operations are indivisible, so interleaving cannot occur between read and write steps. This prevents subtle races by guaranteeing that updates to shared state happen as a single, uninterruptible operation.

Overall interpretation: The examples illustrate how unsynchronized concurrent access to shared state can produce incorrect results, and the techniques listed provide ways to serialize or atomicize those accesses to maintain correctness. While other approaches (e.g., transactional memory, barriers, or lock-free data structures) exist, the three scenarios identify common causes, and the three techniques cover foundational methods to mitigate racers in typical multi-threaded or multi-process environments.

b) Describe three scenarios that can lead to race conditions and discuss three techniques to prevent them. (15 marks)论述题

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