spinlocks - TarisMajor/5143-OpSystems GitHub Wiki

Spinlocks are a type of synchronization primitive used in concurrent programming to ensure mutual exclusion. Unlike other locking mechanisms that put the thread to sleep while waiting for the lock, spinlocks continuously check if the lock is available, thus "spinning" in a loop until they can acquire the lock. Spinlocks are typically used in scenarios where the wait time is expected to be very short.

1. Busy Waiting: Spinlocks rely on busy waiting, where a thread repeatedly checks for the lock without yielding the CPU.
2. Efficient for Short Waits: Spinlocks are efficient when the critical section is expected to be held for a very short duration, as the overhead of putting the thread to sleep and waking it up can be higher than busy waiting.
3. Non-Preemptive: Spinlocks assume that the lock will be held briefly and that the system will not switch out the spinning thread, making them suitable for multiprocessor systems.

1. Low Overhead: For short wait times, spinlocks have lower overhead compared to putting a thread to sleep and waking it up.
2. Simplicity: Spinlocks are simple to implement and use, making them suitable for low-level synchronization in kernel and driver code.
3. High Performance on Multiprocessor Systems: Spinlocks perform well on multiprocessor systems where one processor can hold the lock while another spins.

1. Inefficiency for Long Waits: If the critical section is held for a long time, spinlocks can waste CPU cycles by spinning, making them inefficient.
2. Not Suitable for Uniprocessor Systems: Spinlocks are less effective on uniprocessor systems, where the spinning thread cannot proceed until the current thread releases the lock.
3. Priority Inversion: Spinlocks can cause priority inversion, where a higher-priority thread spins while waiting for a lower-priority thread to release the lock.

1. Kernel Programming: Spinlocks are often used in kernel programming for low-level synchronization, such as in operating system kernels and device drivers.
2. Short Critical Sections: Spinlocks are suitable for protecting short critical sections where the lock hold time is minimal.
3. Multiprocessor Systems: Spinlocks are effective in multiprocessor systems where multiple processors can work in parallel and the wait time for the lock is expected to be short.

Consider a simple implementation of a spinlock in C:

Pseudocode:

typedef struct {
    volatile int lock;
} spinlock_t;

void spinlock_init(spinlock_t *lock) {
    lock->lock = 0;
}

void spinlock_acquire(spinlock_t *lock) {
    while (__sync_lock_test_and_set(&lock->lock, 1)) {
        // Busy wait (spin) until the lock is released
    }
}

void spinlock_release(spinlock_t *lock) {
    __sync_lock_release(&lock->lock);
}

In this example, spinlock_acquire uses an atomic test-and-set operation to acquire the lock. If the lock is already held, it spins in a loop until the lock becomes available. spinlock_release releases the lock by resetting it.

1. Mutexes: Unlike spinlocks, mutexes put the thread to sleep while waiting for the lock, reducing CPU usage but introducing context-switching overhead.
2. Semaphores: Semaphores can control access to a resource by multiple threads, allowing for more complex synchronization patterns.
3. Condition Variables: Used for synchronization where threads need to wait for specific conditions to be met.

• Operating System Concepts by Abraham Silberschatz, Peter B. Galvin, and Greg Gagne
• Spinlocks - GeeksforGeeks
• Modern Operating Systems by Andrew S. Tanenbaum
• Wikipedia - Spinlock
• ChatGPT