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fixups for memory model document

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R=rsc
CC=go-dev
http://go/go-review/1016018
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Rob Pike 2009-11-01 20:58:33 -08:00
parent 6ac19ecefa
commit 4e5296d4ba
1 changed files with 32 additions and 44 deletions
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@ -15,16 +15,17 @@ 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.
does not change the behavior within that goroutine
as defined by the language specification.
Because of this reordering, the execution order observed
by one may differ from the order perceived
by one goroutine may differ from the order perceived
by another. For example, if one goroutine
executes <code>a = 1; b = 2;</code>, a second goroutine might observe
executes <code>a = 1; b = 2;</code>, another might observe
the updated value of <code>b</code> before the updated value of <code>a</code>.
</p>
<p>
To specify the requirements on reads and writes, we define
To specify the requirements of reads and writes, we define
<i>happens before</i>, a partial order on the execution
of memory operations in a Go program. If event <span class="event">e<sub>1</sub></span> happens
before event <span class="event">e<sub>2</sub></span>, then we say that <span class="event">e<sub>2</sub></span> happens after <span class="event">e<sub>1</sub></span>.
@ -34,7 +35,7 @@ after <span class="event">e<sub>2</sub></span>, then we say that <span class="ev
<p>
Within a single goroutine, the happens before order is the
order specified by the program.
order expressed by the program.
</p>
<p>
@ -93,7 +94,7 @@ unspecified order.
<h3>Initialization</h3>
<p>
Program initialization runs in a single goroutine, and
Program initialization runs in a single goroutine and
new goroutines created during initialization do not
start running until initialization ends.
</p>
@ -157,12 +158,12 @@ receive from that channel completes.
</p>
<p>
For example, this program:
This program:
</p>
<pre>
var c = make(chan int, 10);
var a string;
var c = make(chan int, 10)
var a string
func f() {
a = "hello, world";
@ -189,12 +190,12 @@ the send on that channel completes.
</p>
<p>
For example, this program:
This program:
</p>
<pre>
var c = make(chan int);
var a string;
var c = make(chan int)
var a string
func f() {
a = "hello, world";
@ -211,7 +212,7 @@ func main() {
</pre>
<p>
is also guaranteed to print "hello, world". The write to <code>a</code>
is also guaranteed to print <code>"hello, world"</code>. The write to <code>a</code>
happens before the receive on <code>c</code>, which happens before
the corresponding send on <code>c</code> completes, which happens
before the <code>print</code>.
@ -237,12 +238,12 @@ the <i>n</i>'th call to <code>l.Unlock()</code> happens before the <i>m</i>'th c
</p>
<p>
For example, this program:
This program:
</p>
<pre>
var l sync.Mutex;
var a string;
var l sync.Mutex
var a string
func f() {
a = "hello, world";
@ -278,16 +279,16 @@ but only one will run <code>f()</code>, and the other calls block
until <code>f()</code> has returned.
</p>
<p>
A single call to <code>f()</code> happens before <code>once.Do(f)</code> returns.
<p class="rule">
A single call of <code>f()</code> from <code>once.Do(f)</code> happens (returns) before any call of <code>once.Do(f)</code> returns.
</p>
<p>
For example, in this program:
In this program:
</p>
<pre>
var a string;
var a string
func setup() {
a = "hello, world";
@ -319,11 +320,11 @@ will observe writes that happened before <span class="event">w</span>.
</p>
<p>
For example, in this program:
In this program:
</p>
<pre>
var a, b int;
var a, b int
func f() {
a = 1;
@ -346,18 +347,18 @@ it can happen that <code>g</code> prints <code>2</code> and then <code>0</code>.
</p>
<p>
This fact invalidates a few obvious idioms.
This fact invalidates a few common idioms.
</p>
<p>
Double-checked locking is an attempt to avoid the overhead of synchronization.
For example, the <code>twoprint</code> program above, might be
For example, the <code>twoprint</code> program might be
incorrectly written as:
</p>
<pre>
var a string;
var done bool;
var a string
var done bool
func setup() {
a = "hello, world";
@ -389,8 +390,8 @@ Another incorrect idiom is busy waiting for a value, as in:
</p>
<pre>
var a string;
var done bool;
var a string
var done bool
func setup() {
a = "hello, world";
@ -407,7 +408,7 @@ func main() {
<p>
As before, there is no guarantee that, in <code>main</code>,
observing of the write to <code>done</code>
observing the write to <code>done</code>
implies observing the write to <code>a</code>, so this program could
print an empty string too.
Worse, there is no guarantee that the write to <code>done</code> will ever
@ -417,7 +418,7 @@ guaranteed to finish.
</p>
<p>
There are subtler variants on this theme. For example, in this program:
There are subtler variants on this theme, such as this program.
</p>
<pre>
@ -425,7 +426,7 @@ type T struct {
msg string;
}
var g *T;
var g *T
func setup() {
t := new(T);
@ -451,16 +452,3 @@ value for <code>g.msg</code>.
In all these examples, the solution is the same:
use explicit synchronization.
</p>
</div>
<div id="footer">
<p>Except as noted, this content is
licensed under <a href="http://creativecommons.org/licenses/by/3.0/">
Creative Commons Attribution 3.0</a>.
</div>
</body>
</html>