The Go Memory Model

The Go Memory Model

Introduction

Happens Before

Synchronization

Initialization

Goroutine creation

Channel communication

Locks

Once

Incorrect synchronization

Introduction

The Go memory model specifies the conditions under which reads of a variable in one goroutine can be guaranteed to observe values produced by writes to the same variable in a different goroutine.

Happens Before

Within a single goroutine, reads and writes must behave as if they executed in the order specified by the program. That is, compilers and processors may reorder the reads and writes executed within a single goroutine only when the reordering does not change the behavior within that goroutine as defined by the language specification. Because of this reordering, the execution order observed by one goroutine may differ from the order perceived by another. For example, if one goroutine executes a = 1; b = 2;, another might observe the updated value of b before the updated value of a.

To specify the requirements of reads and writes, we define happens before, a partial order on the execution of memory operations in a Go program. If event e1 happens before event e2, then we say that e2 happens after e1. Also, if e1 does not happen before e2 and does not happen after e2, then we say that e1 and e2 happen concurrently.

Within a single goroutine, the happens before order is the order expressed by the program.

A read r of a variable v is allowed to observe a write w to v if both of the following hold:

1. w happens before r.

2. There is no other write w' to v that happens after w but before r.

To guarantee that a read r of a variable v observes a particular write w to v, ensure that w is the only write r is allowed to observe. That is, r is guaranteed to observe w if both of the following hold:

1. w happens before r.

2. Any other write to the shared variable v either happens before w or after r.

This pair of conditions is stronger than the first pair; it requires that there are no other writes happening concurrently with w or r.

Within a single goroutine, there is no concurrency, so the two definitions are equivalent: a read r observes the value written by the most recent write w to v. When multiple goroutines access a shared variable v, they must use synchronization events to establish happens-before conditions that ensure reads observe the desired writes.

The initialization of variable v with the zero value for v's type behaves as a write in the memory model.

Reads and writes of values larger than a single machine word behave as multiple machine-word-sized operations in an unspecified order.

Synchronization

Initialization

Program initialization runs in a single goroutine and new goroutines created during initialization do not start running until initialization ends.

If a package p imports package q, the completion of q's init functions happens before the start of any of p's.

The start of the function main.main happens after all init functions have finished.

The execution of any goroutines created during init functions happens after all init functions have finished.

Goroutine creation

The go statement that starts a new goroutine happens before the goroutine's execution begins.

For example, in this program:

var a string;

func f() {

print(a);

}

func hello() {

a = "hello, world";

go f();

}

calling hello will print "hello, world" at some point in the future (perhaps after hello has returned).

Channel communication

Channel communication is the main method of synchronization between goroutines. Each send on a particular channel is matched to a corresponding receive from that channel, usually in a different goroutine.

A send on a channel happens before the corresponding receive from that channel completes.

This program:

var c = make(chan int, 10)

var a string

func f() {

a = "hello, world";

c <- 0;

}

func main() {

go f();

<-c;

print(a);

}

is guaranteed to print "hello, world". The write to a happens before the send on c, which happens before the corresponding receive on c completes, which happens before the print.

A receive from an unbuffered channel happens before the send on that channel completes.

This program:

var c = make(chan int)

var a string

func f() {

a = "hello, world";

<-c;

}

func main() {

go f();

c <- 0;

print(a);

}

is also guaranteed to print "hello, world". The write to a happens before the receive on c, which happens before the corresponding send on c completes, which happens before the print.

If the channel were buffered (e.g., c = make(chan int, 1)) then the program would not be guaranteed to print "hello, world". (It might print the empty string; it cannot print "hello, sailor", nor can it crash.)

Locks

The sync package implements two lock data types, sync.Mutex and sync.RWMutex.

For any sync.Mutex or sync.RWMutex variable l and n < m, the n'th call to l.Unlock() happens before the m'th call to l.Lock() returns.

This program:

var l sync.Mutex

var a string

func f() {

a = "hello, world";

l.Unlock();

}

func main() {

l.Lock();

go f();

l.Lock();

print(a);

}

is guaranteed to print "hello, world". The first call to l.Unlock() (in f) happens before the second call to l.Lock() (in main) returns, which happens before the print.

For any call to l.RLock on a sync.RWMutex variable l, there is an n such that the l.RLock happens (returns) after the n'th call to l.Unlock and the matching l.RUnlock happens before the n+1'th call to l.Lock.

Once

The once package provides a safe mechanism for initialization in the presence of multiple goroutines. Multiple threads can execute once.Do(f) for a particular f, but only one will run f(), and the other calls block until f() has returned.

A single call of f() from once.Do(f) happens (returns) before any call of once.Do(f) returns.

In this program:

var a string