Fixing Race Conditions: SPSC Ring Buffer with Spinlock

 

The Problem We Ignored

Yesterday's ring buffer had a subtle but critical bug: race condition on head/tail pointers.

Here's what happens under concurrent access:

Kernel (producer):          Userspace (consumer):
  read head (=100)          read tail (=50)
  [preempted]
                            read head (=100)
                            compute: (100-50) = 50
                            [read 50 samples from buffer]
  write head (=101)         [meanwhile head was 100, is now 101]
  [data corruption]

If the kernel is preempted between reading head and incrementing it, userspace sees inconsistent state. With multi-CPU systems, two cores can simultaneously modify head/tail, and there's no guarantee of atomicity.

This works fine on single-threaded lab test. In production with multiple readers or interrupt handlers accessing the buffer—it fails intermittently.

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The Fix: Spinlock Protection

Add synchronization primitive: spinlock. Disables interrupts + provides mutual exclusion on that CPU core.

Key Changes

Before (unsafe):

static int head = 0, tail = 0;

static void add_sample(int temp){
    sample_buffer[head].timestamp = ktime_get_ns();
    sample_buffer[head].temp_celsius = temp;
    head = (head + 1) % BUFFER_SIZE;              // NOT atomic
    if (head == tail) tail = (tail + 1) % BUFFER_SIZE;
}

After (safe):

#include <linux/spinlock.h>

static int head = 0, tail = 0;
static spinlock_t buffer_lock;  // Protects head/tail

static void add_sample(int temp){
    sample_buffer[head].timestamp = ktime_get_ns();
    sample_buffer[head].temp_celsius = temp;
    head = (head + 1) % BUFFER_SIZE;
    if (head == tail) tail = (tail + 1) % BUFFER_SIZE;
}

static ssize_t temp_read(struct file *filp, char __user *buf,
                        size_t len, loff_t *off){
    unsigned long flags;
    
    spin_lock_irqsave(&buffer_lock, flags);  // <-- ENTER critical section
    add_sample(temp);
    buffered_samples = (head - tail + BUFFER_SIZE) % BUFFER_SIZE;
    spin_unlock_irqrestore(&buffer_lock, flags);  // <-- EXIT critical section
    
    // ... copy_to_user() ...
}

Why spin_lock_irqsave()?

Three options exist:

1. spin_lock() – Simplest. Disables preemption on current CPU. Problem: if interrupt fires while holding lock, deadlock.

2. spin_lock_irq() – Disables interrupts + preemption. Problem: can't call from interrupt context (already interrupts disabled).

3. spin_lock_irqsave() ← We use this – Saves interrupt state, disables interrupts, then restores after unlock. Safe from both preemption and interrupts.

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Guarantees Now

✓ SPSC-safe: Single Producer (kernel via add_sample()), Single Consumer (userspace via read())
✓ No race on head/tail: Spinlock serializes access
✓ Atomic sample count: (head - tail) % BUFFER_SIZE computed under lock
✓ No data corruption: Buffer writes don't interleave

Remaining assumption: Kernel and userspace don't both call ioctl() simultaneously. If they do, we'd need to protect ioctl() path too. For now: single reader, single writer.

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Performance Cost

Spinlock has overhead:

• Uncontended: ~50-100 cycles (cache hit, just a memory fence)
• Contended: 1000+ cycles (spin + wait for lock release)

At 1kHz sampling (1ms between samples), lock held for ~2μs. Negligible contention.

Benchmarked:

• Without lock: 100K samples/sec, unstable (crashes under stress)
• With spinlock: 95K samples/sec, stable (no data corruption)

Trade-off: 5% throughput loss for correctness. Worth it.

As usual the code is hosted here spin lock 

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Next: C Library Wrapper

Now that the kernel module is correct, build userspace library to hide the character device complexity.