Btukfyl

what is the difference between RLock() and Lock() in Golang and how they can be used efficiently when we use mutex Lock ?

Lock(): only one go routine read/write at a time by acquiring the lock.

RLock(): multiple go routine can read(not write) at a time by acquiring the lock.

package main



import (

    "fmt"

    "sync"

    "time"

)



func main() {



    a := 0



    lock := sync.RWMutex{}



    for i := 1; i < 10; i++ {

        go func(i int) {

            lock.Lock()

            fmt.Printf("Lock: from go routine %d: a = %dn",i, a)

            time.Sleep(time.Second)

            lock.Unlock()

        }(i)

    }



    b := 0



    for i := 11; i < 20; i++ {

        go func(i int) {

            lock.RLock()

            fmt.Printf("RLock: from go routine %d: b = %dn",i, b)

            time.Sleep(time.Second)

            lock.RUnlock()

        }(i)

    }



    <-time.After(time.Second*10)

}

1) When a go-routine has already acquired a RLock(), can another go-routine acquire a Lock() for write or it has to wait until RUnlock() happens?

• To acquire a Lock() for write it has to wait until RUnlock()

2) What happens when someone already acquired Lock() for map ,will other go-routine can still get RLock()

• if someone X already acquired Lock(), then other go-routine to get RLock() will have to wait until X release lock (Unlock())

3) Assuming we are dealing with Maps here, is there any possibility of "concurrent read/write of Map" error can come?

• Map is not thread safe. so "concurrent read/write of Map" can cause error.

See following example for more clarification:

package main



import (

    "fmt"

    "sync"

    "time"

)



func main() {

    lock := sync.RWMutex{}



    b := map[string]int{}

    b["0"] = 0



    go func(i int) {

        lock.RLock()

        fmt.Printf("RLock: from go routine %d: b = %dn",i, b["0"])

        time.Sleep(time.Second*3)

        fmt.Printf("RLock: from go routine %d: lock releasedn",i)

        lock.RUnlock()

    }(1)



    go func(i int) {

        lock.Lock()

        b["2"] = i

        fmt.Printf("Lock: from go routine %d: b = %dn",i, b["2"])

        time.Sleep(time.Second*3)

        fmt.Printf("Lock: from go routine %d: lock releasedn",i)

        lock.Unlock()

    }(2)



    <-time.After(time.Second*8)



    fmt.Println("*************************************8")



    go func(i int) {

        lock.Lock()

        b["3"] = i

        fmt.Printf("Lock: from go routine %d: b = %dn",i, b["3"])

        time.Sleep(time.Second*3)

        fmt.Printf("Lock: from go routine %d: lock releasedn",i)

        lock.Unlock()

    }(3)



    go func(i int) {

        lock.RLock()

        fmt.Printf("RLock: from go routine %d: b = %dn",i, b["3"])

        time.Sleep(time.Second*3)

        fmt.Printf("RLock: from go routine %d: lock releasedn",i)

        lock.RUnlock()

    }(4)



    <-time.After(time.Second*8)

}

A RWMutex is a reader/writer mutual exclusion lock. The lock can be held by an arbitrary number of readers or a single writer. The zero value for a RWMutex is an unlocked mutex.

A RWMutex must not be copied after first use.

If a goroutine holds a RWMutex for reading and another goroutine might call Lock, no goroutine should expect to be able to acquire a read lock until the initial read lock is released. In particular, this prohibits recursive read locking. This is to ensure that the lock eventually becomes available; a blocked Lock call excludes new readers from acquiring the lock.

A Mutex is a mutual exclusion lock. The zero value for a Mutex is an unlocked mutex.

The golang provide the channel is the best practice for concurrency control, so i think the efficiently way using sync.lock is not used it, use channel instead.