The ABA Problem and Solutions
One of the challenges in lock-free programming is the ABA problem, where a value changes from A to B and back to A, potentially causing incorrect behavior in compare-and-swap operations:
package main
import (
"fmt"
"sync"
"sync/atomic"
"time"
)
func demonstrateABAProblem() {
// Simple stack implementation using a linked list
type node struct {
value int
next *node
}
// Head pointer
var head atomic.Pointer[node]
// Initialize with some values
nodeC := &node{value: 3}
nodeB := &node{value: 2, next: nodeC}
nodeA := &node{value: 1, next: nodeB}
head.Store(nodeA)
// Function to pop a node
pop := func() *node {
for {
oldHead := head.Load()
if oldHead == nil {
return nil
}
newHead := oldHead.next
if head.CompareAndSwap(oldHead, newHead) {
return oldHead
}
}
}
// Function to push a node
push := func(n *node) {
for {
oldHead := head.Load()
n.next = oldHead
if head.CompareAndSwap(oldHead, n) {
return
}
}
}
// Simulate the ABA problem
var wg sync.WaitGroup
wg.Add(2)
// Goroutine 1: Pop two nodes, then push the first one back
go func() {
defer wg.Done()
// Pop nodeA
nodeA := pop()
fmt.Printf("Goroutine 1: Popped %d\n", nodeA.value)
// Simulate some delay
time.Sleep(100 * time.Millisecond)
// Push nodeA back
push(nodeA)
fmt.Printf("Goroutine 1: Pushed %d back\n", nodeA.value)
}()
// Goroutine 2: Pop a node, then push two new nodes, then pop again
go func() {
defer wg.Done()
// Wait for Goroutine 1 to pop nodeA
time.Sleep(10 * time.Millisecond)
// Pop nodeB
nodeB := pop()
fmt.Printf("Goroutine 2: Popped %d\n", nodeB.value)
// Pop nodeC
nodeC := pop()
fmt.Printf("Goroutine 2: Popped %d\n", nodeC.value)
// Push new nodes
push(&node{value: 4})
push(&node{value: 5})
fmt.Printf("Goroutine 2: Pushed 4 and 5\n")
}()
wg.Wait()
// Print the final stack
fmt.Println("Final stack:")
for curr := head.Load(); curr != nil; curr = curr.next {
fmt.Printf("%d ", curr.value)
}
fmt.Println()
}
// Solution to ABA: Use version counters (tagged pointers)
func abaWithVersionCounters() {
// Node with version counter
type versionedNode struct {
node *node
version uint64
}
type node struct {
value int
next *node
}
// Head pointer with version
var head atomic.Uint64
var nodes []*node
// Initialize with some values
nodes = append(nodes, &node{value: 1})
nodes = append(nodes, &node{value: 2})
nodes = append(nodes, &node{value: 3})
// Link the nodes
nodes[0].next = nodes[1]
nodes[1].next = nodes[2]
// Store the initial head (index 0, version 1)
head.Store(1) // Version 1, index 0
// Function to pop a node
pop := func() *node {
for {
oldHead := head.Load()
oldVersion := oldHead >> 32
oldIndex := oldHead & 0xFFFFFFFF
if oldIndex >= uint64(len(nodes)) {
return nil
}
oldNode := nodes[oldIndex]
var newIndex uint64
if oldNode.next != nil {
// Find the index of the next node
for i, n := range nodes {
if n == oldNode.next {
newIndex = uint64(i)
break
}
}
} else {
newIndex = 0xFFFFFFFF // Special value for nil
}
// Increment version
newVersion := oldVersion + 1
newHead := (newVersion << 32) | newIndex
if head.CompareAndSwap(oldHead, newHead) {
return oldNode
}
}
}
// Function to push a node
push := func(value int) {
for {
oldHead := head.Load()
oldVersion := oldHead >> 32
oldIndex := oldHead & 0xFFFFFFFF
var oldNode *node
if oldIndex < uint64(len(nodes)) {
oldNode = nodes[oldIndex]
}
// Create new node
newNode := &node{value: value, next: oldNode}
nodes = append(nodes, newNode)
newIndex := uint64(len(nodes) - 1)
// Increment version
newVersion := oldVersion + 1
newHead := (newVersion << 32) | newIndex
if head.CompareAndSwap(oldHead, newHead) {
return
}
}
}
// Test the versioned stack
fmt.Println("Testing versioned stack (ABA solution):")
// Pop the first node
node1 := pop()
fmt.Printf("Popped: %d\n", node1.value)
// Pop the second node
node2 := pop()
fmt.Printf("Popped: %d\n", node2.value)
// Push the first node back (would cause ABA in a naive implementation)
push(node1.value)
fmt.Printf("Pushed: %d\n", node1.value)
// Push a new node
push(4)
fmt.Printf("Pushed: %d\n", 4)
// Print the final stack
fmt.Println("Final stack:")
for curr := head.Load(); curr != 0xFFFFFFFF; {
version := curr >> 32
index := curr & 0xFFFFFFFF
if index >= uint64(len(nodes)) {
break
}
node := nodes[index]
fmt.Printf("%d (v%d) ", node.value, version)
// Find the next node's index
var nextIndex uint64 = 0xFFFFFFFF
if node.next != nil {
for i, n := range nodes {
if n == node.next {
nextIndex = uint64(i)
break
}
}
}
curr = (version << 32) | nextIndex
}
fmt.Println()
}
func main() {
fmt.Println("Demonstrating the ABA problem:")
demonstrateABAProblem()
fmt.Println("\nDemonstrating ABA solution with version counters:")
abaWithVersionCounters()
}
This example demonstrates the ABA problem and a solution using version counters. The ABA problem occurs when a thread reads a value A, gets preempted, and then another thread changes the value to B and back to A. When the first thread resumes, it cannot detect that the value has changed. The solution is to use version counters or “tagged pointers” that increment with each modification, ensuring that even if the pointer value is the same, the version will be different.
Compare-and-Swap Patterns
Compare-and-swap (CAS) operations are the foundation of lock-free programming. Let’s explore common patterns and techniques for using CAS effectively.
Basic CAS Loop Pattern
The most common pattern in lock-free programming is the CAS loop:
package main
import (
"fmt"
"sync"
"sync/atomic"
)
func casLoopPattern() {
var value atomic.Int64
value.Store(10)
// Basic CAS loop pattern
updateValue := func(delta int64) {
for {
// Read the current value
current := value.Load()
// Compute the new value
new := current + delta
// Try to update the value
if value.CompareAndSwap(current, new) {
// Success! Value was updated
return
}
// If CAS failed, another goroutine modified the value
// Loop and try again
}
}
// Test the CAS loop
fmt.Printf("Initial value: %d\n", value.Load())
updateValue(5)
fmt.Printf("After +5: %d\n", value.Load())
updateValue(-3)
fmt.Printf("After -3: %d\n", value.Load())
// Test concurrent updates
var wg sync.WaitGroup
for i := 0; i < 100; i++ {
wg.Add(1)
go func() {
defer wg.Done()
updateValue(1)
}()
}
wg.Wait()
fmt.Printf("After 100 concurrent +1 operations: %d\n", value.Load())
}
func main() {
casLoopPattern()
// Demonstrate more advanced patterns
backoffPattern()
conditionalUpdate()
multipleFieldUpdate()
}
This example demonstrates the basic CAS loop pattern, which is the foundation of most lock-free algorithms. The pattern involves reading the current value, computing a new value based on the current one, and then attempting to update the value using CAS. If the CAS fails, the loop retries with the updated current value.
Exponential Backoff Pattern
In high-contention scenarios, adding a backoff strategy can improve performance:
func backoffPattern() {
var value atomic.Int64
// CAS loop with exponential backoff
updateWithBackoff := func(delta int64) {
backoff := 1 // Start with minimal backoff
for {
// Read the current value
current := value.Load()
// Compute the new value
new := current + delta
// Try to update the value
if value.CompareAndSwap(current, new) {
// Success! Value was updated
return
}
// If CAS failed, apply backoff
for i := 0; i < backoff; i++ {
// Simple busy-wait backoff
// In real code, consider runtime.Gosched() or time.Sleep
}
// Increase backoff exponentially, up to a maximum
if backoff < 1000 {
backoff *= 2
}
}
}
// Test concurrent updates with backoff
var wg sync.WaitGroup
start := time.Now()
for i := 0; i < 1000; i++ {
wg.Add(1)
go func() {
defer wg.Done()
updateWithBackoff(1)
}()
}
wg.Wait()
duration := time.Since(start)
fmt.Printf("\nBackoff pattern:\n")
fmt.Printf(" 1000 concurrent updates with backoff: %d\n", value.Load())
fmt.Printf(" Duration: %v\n", duration)
}
This example demonstrates a CAS loop with exponential backoff. When contention is high, adding a backoff strategy can reduce the number of failed CAS operations and improve overall throughput.