Reason About Deadlocks, Synchronization, False Sharing, and Privilege Transitions

Overview

In storage and operating system development, low-level concurrency and CPU-memory interactions directly dictate system reliability and throughput. Review the three scenarios below and provide technical answers distinguishing synchronization primitives, hardware cache behavior, and OS privilege transitions.

Scenario 1: Deadlock and Lock Ordering

Consider two worker threads running the following routines on shared resources protected by mutexes lock_x and lock_y:

void worker_transfer_forward() {
    pthread_mutex_lock(&lock_x);
    pthread_mutex_lock(&lock_y);
    /* perform transfer X -> Y */
    pthread_mutex_unlock(&lock_y);
    pthread_mutex_unlock(&lock_x);
}

void worker_transfer_backward() {
    pthread_mutex_lock(&lock_y);
    pthread_mutex_lock(&lock_x);
    /* perform transfer Y -> X */
    pthread_mutex_unlock(&lock_x);
    pthread_mutex_unlock(&lock_y);
}

1. Name the four Coffman conditions for deadlocks. Which specific condition is violated or present here that triggers the hazard?
2. Provide the standard design pattern to eliminate this deadlock without reducing the lock granularity.

Scenario 2: Cache Performance and Primitives

Two threads execute on distinct physical CPU cores. Each thread frequently increments its own 64-bit performance counter:

struct ThreadStats {
    uint64_t thread_0_ops; // modified exclusively by Thread 0
    uint64_t thread_1_ops; // modified exclusively by Thread 1
} stats;

1. Explain why this design suffers severe throughput degradation even though threads access entirely separate member variables without lock contention.
2. How would you resolve this issue in C/C++?
3. When would you prefer a spinlock over a mutex for synchronizing shared state, and what hardware/scheduler risk does a spinlock introduce on a single-core system or under preemption?

Scenario 3: Privilege Transitions and Faults

1. Contrast the CPU mechanics of a user-space thread issuing a valid read() system call versus a thread dereferencing an invalid pointer that raises a SIGSEGV.
2. Clearly define the difference between blocking (as in pthread_mutex_lock), busy-waiting (as in pthread_spin_lock), and a hardware protection fault.