sync.Mutex

Learn to synchronize access to shared data with a Mutex.

sync.Mutex

If several goroutines write the same variable at once, a data race happens and the result is unpredictable. sync.Mutex lets only one goroutine run a piece of code at a time:

var mu sync.Mutex
sayac := 0

mu.Lock()
sayac++
mu.Unlock()

The region between Lock and Unlock is called the critical section.

defer Unlock and the race detector

Forgetting to unlock freezes the program (deadlock). The safest pattern is to write defer right after locking:

func (s *Sayac) Artir() {
    s.mu.Lock()
    defer s.mu.Unlock()
    s.n++
}

To find races use go test -race or go run -race. A struct containing a mutex must not be copied, which is why its methods use pointer receivers.

Exercise Safe counter

Write type Sayac struct { mu sync.Mutex; n int } and a locked Artir() method. Each of 100 goroutines calls Artir once; wait with a sync.WaitGroup and print n.

Show example solution

package main

import (
    "fmt"
    "sync"
)

type Sayac struct {
    mu sync.Mutex
    n  int
}

func (s *Sayac) Artir() {
    s.mu.Lock()
    defer s.mu.Unlock()
    s.n++
}

func main() {
    var s Sayac
    var wg sync.WaitGroup
    for i := 0; i < 100; i++ {
        wg.Add(1)
        go func() {
            defer wg.Done()
            s.Artir()
        }()
    }
    wg.Wait()
    fmt.Println(s.n)
}

Test yourself

Why isn't <code>sayac++</code> safe without a lock?
- An int can't be shared
- The compiler doesn't allow it
- Read-increment-write is three steps; another goroutine can interleave
- The ++ operator is slow
Show answer

Read-increment-write is three steps; another goroutine can interleave — If two goroutines read the same old value and each add one, an increment is lost. go run -race catches this race.

Unlock

s.mu.Lock()
____ s.mu.Unlock()
s.n++

Show answer

s.mu.Lock()
defer s.mu.Unlock()
s.n++

Concept

Data race

Unprotected concurrent access
Critical section

Between Lock and Unlock
Deadlock

A lock never released
-race

Race detector

Next lesson: sync.RWMutex

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