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Mutex

Status

This note is complete, reviewed, and considered stable.

Mutexes are synchronization primitives from the sync package that prevent race conditions when multiple goroutines access shared data. Only one goroutine can hold the lock at a time, ensuring safe access.

Basic Usage​

Wrap shared data access in a critical section:

mu.Lock()
// critical section
mu.Unlock()

Example: Protecting Shared Data​

Without Mutex (race condition)​

package main

import (
    "fmt"
    "time"
)

var counter int

func increment() {
    counter++
}

func main() {
    for i := 0; i < 1000; i++ {
        go increment()
    }
    time.Sleep(time.Second)
    fmt.Println("Final Counter:", counter)
}

// Final counter will likely be less than 1000 due to race conditions.

With Mutex (no race condition)​

package main

import (
    "fmt"
    "sync"
    "time"
)

var counter int
var mu sync.Mutex

func increment() {
    mu.Lock()
    counter++
    mu.Unlock()
}

func main() {
    for i := 0; i < 1000; i++ {
        go increment()
    }
    time.Sleep(time.Second)
    fmt.Println("Final Counter:", counter)
}

// Output will always be Final Counter: 1000

Best Practices​

Example: Using defer to Unlock​

func increment() {
    mu.Lock()
    defer mu.Unlock()
    counter++
}

// Ensures mutex is always unlocked, even on panic

Multiple Mutexes & Deadlocks​

Always lock multiple mutexes in the same order to avoid deadlocks.

var mu1 sync.Mutex
var mu2 sync.Mutex

func routine1() {
    mu1.Lock()
    time.Sleep(100 * time.Millisecond)
    mu2.Lock()
    fmt.Println("Routine 1 finished")
    mu2.Unlock()
    mu1.Unlock()
}

func routine2() {
    mu2.Lock()
    time.Sleep(100 * time.Millisecond)
    mu1.Lock()
    fmt.Println("Routine 2 finished")
    mu1.Unlock()
    mu2.Unlock()
}

func main() {
    go routine1()
    go routine2()
    time.Sleep(time.Second)
}

// routine1 locks mu1 then mu2, routine2 locks mu2 then mu1: causes deadlock

Mutex vs RWMutex​

Example​

var rw sync.RWMutex

func readData() {
    rw.RLock()
    fmt.Println("Reading data...")
    time.Sleep(100 * time.Millisecond)
    rw.RUnlock()
}

func writeData() {
    rw.Lock()
    fmt.Println("Writing data...")
    time.Sleep(100 * time.Millisecond)
    rw.Unlock()
}

When to Use Mutex​

We should use a Mutex when:

  • We have shared memory accessed by multiple goroutines.
  • We need to protect data integrity.
  • Performance is more important than waiting for safe access (mutexes introduce some blocking).

We should not use a Mutex when:

  • Each goroutine works on independent data.
  • Channels or message passing fits better into the design (channels often simplify concurrency).