Go Race Conditions & Mutexes

Data Race¶

When 2 go routines are executing concurrently (at the same time) if there is no special handling then it can lead to race conditions.
- These are very difficult to debug.
Race conditions occur because of unsynchronised access to memory.

Example of a race condition

import (
    "fmt"
    "sync"
)

func main() {
    var wg sync.WaitGroup
    gr := 100
    wg.Add(gr * 2)
    value := 0

    // we are not using wg pointers here since there is nothing to pass
    // we use pointers in functions because in go functions are call by value
    for i := 0; i < gr; i++ {
        go func() {
            defer wg.Done()
            value++
        }()
        go func() {
            defer wg.Done()
            value--
        }()
    }

    wg.Wait()
    fmt.Println(value)
}
// value can be anything
// Value of n depends on which go routine finishes first

We can use built in race detector to find race conditions in go.
- To use it we pass it as a flag when running main.go : go run -race main.go

Mutexes¶

Mutex comes from mutual exclusion
This is called explicit synchronisation (from experience we realise this variable can be subjected to race condition) where variable access is protected through synchronisation primitives such as a mutex.

Solving the above race condition using a mutex. This problem could also have been solved by a channel

import (
    "fmt"
    "sync"
)

func main() {
    var m sync.Mutex
    var wg sync.WaitGroup
    gr := 100
    wg.Add(gr * 2)
    value := 0

    // if we were using named functions then we would have to pass pointers to the mutex
    for i := 0; i < gr; i++ {
        go func() {
            defer wg.Done()
            m.Lock()
            value++
            m.Unlock()
        }()
        go func() {
            defer wg.Done()
            m.Lock()
            value--
            m.Unlock()
        }()
    }

    wg.Wait()
    fmt.Println(value)
    // even though we have used mutexes if fmt.Println() was above wg.Wait() then
    // it would have been a race condition
}

Last updated: 2022-06-18