Introduction to Linux Spinlocks and Comparison with Mutexes

In modern multi-core systems, synchronizing access to shared resources is critical. The Linux kernel provides several synchronization primitives—among them, spinlocks and mutexes are widely used. This article explains what spinlocks are, how they work in the Linux kernel, and compares them with mutexes. We also provide concrete examples in C to illustrate their differences in usage and performance.

What Are Linux Spinlocks?

A spinlock is a low-level synchronization mechanism that protects a critical section by “spinning” in a tight loop until the lock becomes available. When a thread or CPU core attempts to acquire a spinlock and finds it already held, it repeatedly tests the lock until it succeeds. Because it does not sleep, a spinning thread consumes CPU cycles during waiting.

In the Linux kernel, a spinlock is typically defined by a type such as spinlock_t and is implemented using architecture-specific atomic operations (for example, using the LOCK prefix on x86). In simple terms, the lock is represented as a binary state (locked/unlocked). When you acquire a spinlock, you set the state to “locked” (usually via an atomic test-and-set), and when you release it, you clear that state.

How Spinlocks Work

When a CPU core enters a critical section protected by a spinlock, it does the following:

1. Atomic Test-and-Set: The core uses an atomic operation (such as xchg or lock bts) to change the lock state from “unlocked” to “locked.”
2. Busy Waiting: If the atomic operation indicates that the lock was already held, the core “spins”—repeatedly checking the lock—until it becomes free.
3. Interrupt Control: In kernel space, spinlocks often disable preemption (and even interrupts with variants like spin_lock_irqsave()) to avoid deadlocks in interrupt context.

Because they are simple and do not incur the overhead of putting a thread to sleep, spinlocks can be very fast when contention is low and the critical section is very short. However, if a spinlock is held for an extended time or if many CPUs contend for it, the busy waiting can waste significant CPU time and even lead to cache thrashing.

For more detailed insight into the inner workings of Linux spinlocks, one can refer to articles such as the Linux Inside guide on spinlocks.

Mutexes vs. Spinlocks: Key Differences

While both spinlocks and mutexes are used to serialize access to shared data, their behavior and performance characteristics differ:

• Waiting Mechanism:
• Usage Context:
• Overhead:

A common rule of thumb is: use a spinlock for very short critical sections in interrupt or SMP contexts (where sleeping is not an option) and use a mutex for longer critical sections or where high contention might occur in user context. For an in-depth performance discussion, see Matklad’s “Mutexes Are Faster Than Spinlocks” and comparisons by other kernel developers.

Concrete Examples

Below are simplified examples to demonstrate both spinlock and mutex usage in a Linux-like environment.

Example 1: Using a Spinlock

Imagine a scenario where a shared counter is updated in an interrupt context. In this example, we disable interrupts on the local CPU while acquiring the spinlock.

#include <linux/spinlock.h>
#include <linux/interrupt.h>

static int shared_counter = 0;
static spinlock_t my_spinlock;

void init_my_spinlock(void)
{
    spin_lock_init(&my_spinlock);
}

void update_counter_spinlock(void)
{
    unsigned long flags;
    /* Disable local interrupts and acquire the spinlock */
    spin_lock_irqsave(&my_spinlock, flags);
    
    /* Critical section: update shared resource */
    shared_counter++;
    
    /* Release the spinlock and restore interrupt state */
    spin_unlock_irqrestore(&my_spinlock, flags);
}
        

Explanation:

• The spin_lock_irqsave() variant is used because this function might be called from contexts where interrupts are enabled.
• If the lock is contended, the core busy-waits until the lock is released.

Example 2: Using a Mutex

Now consider a user-space scenario (or a process context in the kernel) where you need to update a shared resource and it is acceptable to sleep while waiting for the lock.

#include <linux/mutex.h>
#include <linux/slab.h>

static int shared_data = 0;
static DEFINE_MUTEX(my_mutex);

void update_data_mutex(void)
{
    /* Acquire the mutex; this may sleep if the mutex is contended */
    mutex_lock(&my_mutex);
    
    /* Critical section: update shared resource */
    shared_data++;
    
    /* Release the mutex */
    mutex_unlock(&my_mutex);
}
        

Explanation:

• Mutexes are used because this code runs in process context where sleeping is allowed.
• The mutex ensures exclusive access to shared_data and avoids busy-waiting when the lock is held by another thread.

When to Choose Spinlocks or Mutexes

Choosing the right locking primitive depends on several factors:

• Critical Section Length: If the critical section is extremely short (e.g., updating a counter) and may be used in interrupt context, a spinlock is appropriate.
• Context of Execution: For code that might run in interrupt context (where sleeping is disallowed), spinlocks are the only option.
• Contention Level: Under high contention, if the expected wait time is longer than the cost of a context switch, mutexes generally provide better throughput.
• CPU Utilization: Spinlocks waste CPU cycles by busy-waiting; if preserving CPU resources is crucial, mutexes are preferred.

For further detailed comparison and performance evaluation, see discussions on Stack Overflow and benchmarking articles like Matklad’s article.

Conclusion

Linux spinlocks are a fundamental tool for ensuring safe access to shared resources in concurrent environments, especially in interrupt context or on SMP systems. However, because they busy-wait while spinning, they are best suited for very short critical sections. In contrast, mutexes—designed for contexts where sleeping is acceptable—provide better overall performance under high contention by reducing wasted CPU cycles. By understanding these tradeoffs and using concrete examples, kernel developers can choose the most appropriate locking primitive for their needs.