runtime, sync, sync/atomic: document happens-before guarantees

A few of these are copied from the memory model doc.
Many are entirely new, following discussion on #47141.
See https://research.swtch.com/gomm for background.

The rule we are establishing is that each type that is meant
to help synchronize a Go program should document its
happens-before guarantees.

For #50859.

Change-Id: I947c40639b263abe67499fa74f68711a97873a39
Reviewed-on: https://go-review.googlesource.com/c/go/+/381316
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/src/sync/map.go b/src/sync/map.go
index 2fa3253429..ec529e056b 100644
--- a/src/sync/map.go
+++ b/src/sync/map.go
@@ -24,6 +24,13 @@ import (
 // contention compared to a Go map paired with a separate Mutex or RWMutex.
 //
 // The zero Map is empty and ready for use. A Map must not be copied after first use.
+//
+// In the terminology of the Go memory model, Map arranges that a write operation
+// “synchronizes before” any read operation that observes the effect of the write, where
+// read and write operations are defined as follows.
+// Load, LoadAndDelete, LoadOrStore are read operations;
+// Delete, LoadAndDelete, and Store are write operations;
+// and LoadOrStore is a write operation when it returns loaded set to false.
 type Map struct {
 	mu Mutex