runtime: add comments to race annotations

Change-Id: Icfb68e73ac38d0a0acc0cda1e41f9e9c5b75ecf5
Reviewed-on: https://go-review.googlesource.com/58110
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/race.go

@@ -4,14 +4,14 @@
 
 // +build race
 
-// Public race detection API, present iff build with -race.
-
 package runtime
 
 import (
 	"unsafe"
 )
 
+// Public race detection API, present iff build with -race.
+
 func RaceRead(addr unsafe.Pointer)
 func RaceWrite(addr unsafe.Pointer)
 func RaceReadRange(addr unsafe.Pointer, len int)
@@ -23,7 +23,69 @@ func RaceErrors() int {
 	return int(n)
 }
 
-// private interface for the runtime
+//go:nosplit
+
+// RaceAcquire/RaceRelease/RaceReleaseMerge establish happens-before relations
+// between goroutines. These inform the race detector about actual synchronization
+// that it can't see for some reason (e.g. synchronization within RaceDisable/RaceEnable
+// sections of code).
+// RaceAcquire establishes a happens-before relation with the preceeding
+// RaceReleaseMerge on addr up to and including the last RaceRelease on addr.
+// In terms of the C memory model (C11 §5.1.2.4, §7.17.3),
+// RaceAcquire is equivalent to atomic_load(memory_order_acquire).
+func RaceAcquire(addr unsafe.Pointer) {
+	raceacquire(addr)
+}
+
+//go:nosplit
+
+// RaceRelease performs a release operation on addr that
+// can synchronize with a later RaceAcquire on addr.
+//
+// In terms of the C memory model, RaceRelease is equivalent to
+// atomic_store(memory_order_release).
+func RaceRelease(addr unsafe.Pointer) {
+	racerelease(addr)
+}
+
+//go:nosplit
+
+// RaceReleaseMerge is like RaceRelease, but also establishes a happens-before
+// relation with the preceeding RaceRelease or RaceReleaseMerge on addr.
+//
+// In terms of the C memory model, RaceReleaseMerge is equivalent to
+// atomic_exchange(memory_order_release).
+func RaceReleaseMerge(addr unsafe.Pointer) {
+	racereleasemerge(addr)
+}
+
+//go:nosplit
+
+// RaceDisable disables handling of race synchronization events in the current goroutine.
+// Handling is re-enabled with RaceEnable. RaceDisable/RaceEnable can be nested.
+// Non-synchronization events (memory accesses, function entry/exit) still affect
+// the race detector.
+func RaceDisable() {
+	_g_ := getg()
+	if _g_.raceignore == 0 {
+		racecall(&__tsan_go_ignore_sync_begin, _g_.racectx, 0, 0, 0)
+	}
+	_g_.raceignore++
+}
+
+//go:nosplit
+
+// RaceEnable re-enables handling of race events in the current goroutine.
+func RaceEnable() {
+	_g_ := getg()
+	_g_.raceignore--
+	if _g_.raceignore == 0 {
+		racecall(&__tsan_go_ignore_sync_end, _g_.racectx, 0, 0, 0)
+	}
+}
+
+// Private interface for the runtime.
+
 const raceenabled = true
 
 // For all functions accepting callerpc and pc,
@@ -433,43 +495,3 @@ func racereleasemergeg(gp *g, addr unsafe.Pointer) {
 func racefingo() {
 	racecall(&__tsan_finalizer_goroutine, getg().racectx, 0, 0, 0)
 }
-
-//go:nosplit
-
-func RaceAcquire(addr unsafe.Pointer) {
-	raceacquire(addr)
-}
-
-//go:nosplit
-
-func RaceRelease(addr unsafe.Pointer) {
-	racerelease(addr)
-}
-
-//go:nosplit
-
-func RaceReleaseMerge(addr unsafe.Pointer) {
-	racereleasemerge(addr)
-}
-
-//go:nosplit
-
-// RaceDisable disables handling of race events in the current goroutine.
-func RaceDisable() {
-	_g_ := getg()
-	if _g_.raceignore == 0 {
-		racecall(&__tsan_go_ignore_sync_begin, _g_.racectx, 0, 0, 0)
-	}
-	_g_.raceignore++
-}
-
-//go:nosplit
-
-// RaceEnable re-enables handling of race events in the current goroutine.
-func RaceEnable() {
-	_g_ := getg()
-	_g_.raceignore--
-	if _g_.raceignore == 0 {
-		racecall(&__tsan_go_ignore_sync_end, _g_.racectx, 0, 0, 0)
-	}
-}