The Go Memory Model

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 execution behavior within that goroutine. Because of this reordering, the execution order observed by one may differ from the order perceived by another. For example, if one goroutine executes a = 1; b = 2;, a second goroutine might observe the updated value of b before the updated value of a.

To specify the requirements on 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 specified 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:

w happens before r.
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:

w happens before r.
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.

For example, 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.

For example, 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 variable l and n < m, the n'th call to l.Unlock() happens before the m'th call to l.Lock() returns.

For example, 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.

TODO(rsc): sync.RWMutex.

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 to f() happens before once.Do(f) returns.

For example, in this program:

var a string;

func setup() {
	a = "hello, world";
}

func doprint() {
	once.Do(setup);
	print(a);
}

func twoprint() {
	go doprint();
	go doprint();
}

calling twoprint causes "hello, world" to be printed twice. The first call to twoprint runs setup once.

Incorrect synchronization

Note that a read r may observe the value written by a write w that happens concurrently with r. Even if this occurs, it does not imply that reads happening after r will observe writes that happened before w.

For example, in this program:

var a, b int;

func f() {
	a = 1;
	b = 2;
}

func g() {
	print(b);
	print(a);
}

func main() {
	go f();
	g();
}

it can happen that g prints 2 and then 0.

This fact invalidates a few obvious idioms.

Double-checked locking is an attempt to avoid the overhead of synchronization. For example, the twoprint program above, might be incorrectly written as:

var a string;
var done bool;

func setup() {
	a = "hello, world";
	done = true;
}

func doprint() {
	if !done {
		once.Do(setup);
	}
	print(a);
}

func twoprint() {
	go doprint();
	go doprint();
}

but there is no guarantee that, in doprint, observing the write to done implies observing the write to a. This version can (incorrectly) print an empty string instead of "hello, world".

Another incorrect idiom is busy waiting for a value, as in:

var a string;
var done bool;

func setup() {
	a = "hello, world";
	done = true;
}

func main() {
	go setup();
	for !done {
	}
	print(a);
}

As before, there is no guarantee that, in main, observing of the write to done implies observing the write to a, so this program could print an empty string too. Worse, there is no guarantee that the write to done will ever be observed by main, since there are no synchronization events between the two threads. The loop in main is not guaranteed to finish.

There are subtler variants on this theme. For example, in this program:

type T struct {
	msg string;
}

var g *T;

func setup() {
	t := new(T);
	t.msg = "hello, world";
	g = t;
}

func main() {
	go setup();
	for g == nil {
	}
	print(g.msg);
}

Even if main observes g != nil and exits its loop, there is no guarantee that it will observe the initialized value for g.msg.

In all these examples, the solution is the same: use explicit synchronization.