When to Use Spin Locks and How They Work in Multithreaded Code

What is a spin lock?

A spin lock is a synchronization primitive in which a thread repeatedly checks (spins) for a lock to become available instead of sleeping. It is useful when the expected wait time is very short, because the thread can acquire the lock immediately once it is released.

pthread spin‑lock API (Linux)

pthread_spinlock_t spin;
pthread_spin_init(&spin, 0);   // initialize (0 = process‑shared)
pthread_spin_lock(&spin);        // acquire (spins until the lock is free)
/* critical section */
pthread_spin_unlock(&spin);      // release
pthread_spin_destroy(&spin);    // destroy

Complete pthread example

#include <pthread.h>
#include <stdio.h>

int shared_data = 0;
pthread_spinlock_t spinlock;

void* increment_data(void* arg) {
    pthread_spin_lock(&spinlock);
    shared_data++;
    printf("Thread %d: shared_data = %d
", *(int*)arg, shared_data);
    pthread_spin_unlock(&spinlock);
    return NULL;
}

int main() {
    pthread_t threads[5];
    int thread_ids[5] = {0,1,2,3,4};
    pthread_spin_init(&spinlock, 0);
    for (int i = 0; i < 5; ++i) {
        pthread_create(&threads[i], NULL, increment_data, &thread_ids[i]);
    }
    for (int i = 0; i < 5; ++i) {
        pthread_join(threads[i], NULL);
    }
    pthread_spin_destroy(&spinlock);
    return 0;
}

Each thread increments shared_data while holding the spin lock, guaranteeing exclusive access.

When to choose a spin lock

Very short critical sections : Updating a small variable or performing a quick read/write where the overhead of putting a thread to sleep would dominate.

Multi‑core systems : One core can spin while other cores continue useful work, improving overall throughput.

Kernel‑level or low‑latency paths : Situations where a context switch is too costly.

If the protected code may run for a noticeable amount of time, a mutex (or other blocking lock) is preferable.

CPU consumption and mitigation

Spinning keeps the CPU busy; prolonged holding of a spin lock can cause high CPU usage. A common mitigation is to limit the number of spin attempts and fall back to a blocking lock.

int try_spinlock_with_limit(pthread_spinlock_t *lock, int max_attempts) {
    int attempt = 0;
    while (attempt < max_attempts) {
        if (pthread_spin_trylock(lock) == 0) {
            return 0; // acquired
        }
        ++attempt;
    }
    return -1; // give up after max_attempts
}

Pros and cons

Advantages

Fast acquisition – no context‑switch overhead.

Works well on multi‑core CPUs where a spinning thread does not block others.

Disadvantages

Consumes CPU cycles while waiting.

Only suitable for short‑duration waits; long waits waste resources.

C++ spin lock using std::atomic_flag

Standard C++ does not provide a spin‑lock type, but a simple spin lock can be built with std::atomic_flag:

#include <atomic>
#include <thread>

class SpinLock {
private:
    std::atomic_flag flag = ATOMIC_FLAG_INIT;
public:
    void lock() {
        while (flag.test_and_set(std::memory_order_acquire)) {
            // busy‑wait
        }
    }
    void unlock() {
        flag.clear(std::memory_order_release);
    }
};

Usage example:

SpinLock spinlock;

void critical_section() {
    spinlock.lock();
    // ... critical code ...
    spinlock.unlock();
}

Conclusion

Spin locks provide low‑latency mutual exclusion for very short critical sections, especially on multi‑core systems or in kernel code. They avoid the overhead of sleeping and waking threads but can waste CPU if the lock is held too long. Use them only when the protected section is guaranteed to be brief, and consider a fallback (e.g., pthread_spin_trylock with a limit) for longer waits.