runtime: add memory barrier for sync send in select

Missed select case when adding the barrier last time.
All the more reason to refactor this code in Go 1.6.

Fixes #11643.

Change-Id: Ib0d19d6e0939296c0a3e06dda5e9b76f813bbc7e
Reviewed-on: https://go-review.googlesource.com/12086
Reviewed-by: Austin Clements <[email protected]>

Author: rsc

src/runtime/chan.go

@@ -165,17 +165,7 @@ func chansend(t *chantype, c *hchan, ep unsafe.Pointer, block bool, callerpc uin
 
 			recvg := sg.g
 			if sg.elem != nil {
-				// This is the only place in the entire runtime where one goroutine
-				// writes to the stack of another goroutine. The GC assumes that
-				// stack writes only happen when the goroutine is running and are
-				// only done by that goroutine. Using a write barrier is sufficient to
-				// make up for violating that assumption, but the write barrier has to work.
-				// typedmemmove will call heapBitsBulkBarrier, but the target bytes
-				// are not in the heap, so that will not help. We arrange to call
-				// memmove and typeBitsBulkBarrier instead.
-				memmove(sg.elem, ep, c.elemtype.size)
-				typeBitsBulkBarrier(c.elemtype, uintptr(sg.elem), c.elemtype.size)
-				sg.elem = nil
+				syncsend(c, sg, ep)
 			}
 			recvg.param = unsafe.Pointer(sg)
 			if sg.releasetime != 0 {
@@ -287,6 +277,21 @@ func chansend(t *chantype, c *hchan, ep unsafe.Pointer, block bool, callerpc uin
 	return true
 }
 
+func syncsend(c *hchan, sg *sudog, elem unsafe.Pointer) {
+	// Send on unbuffered channel is the only operation
+	// in the entire runtime where one goroutine
+	// writes to the stack of another goroutine. The GC assumes that
+	// stack writes only happen when the goroutine is running and are
+	// only done by that goroutine. Using a write barrier is sufficient to
+	// make up for violating that assumption, but the write barrier has to work.
+	// typedmemmove will call heapBitsBulkBarrier, but the target bytes
+	// are not in the heap, so that will not help. We arrange to call
+	// memmove and typeBitsBulkBarrier instead.
+	memmove(sg.elem, elem, c.elemtype.size)
+	typeBitsBulkBarrier(c.elemtype, uintptr(sg.elem), c.elemtype.size)
+	sg.elem = nil
+}
+
 func closechan(c *hchan) {
 	if c == nil {
 		panic("close of nil channel")

src/runtime/chanbarrier_test.go

@@ -0,0 +1,80 @@
+// Copyright 2015 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime_test
+
+import (
+	"runtime"
+	"sync"
+	"testing"
+)
+
+type response struct {
+}
+
+type myError struct {
+}
+
+func (myError) Error() string { return "" }
+
+func doRequest(useSelect bool) (*response, error) {
+	type async struct {
+		resp *response
+		err  error
+	}
+	ch := make(chan *async, 0)
+	done := make(chan struct{}, 0)
+
+	if useSelect {
+		go func() {
+			select {
+			case ch <- &async{resp: nil, err: myError{}}:
+			case <-done:
+			}
+		}()
+	} else {
+		go func() {
+			ch <- &async{resp: nil, err: myError{}}
+		}()
+	}
+
+	r := <-ch
+	runtime.Gosched()
+	return r.resp, r.err
+}
+
+func TestChanSendSelectBarrier(t *testing.T) {
+	testChanSendBarrier(true)
+}
+
+func TestChanSendBarrier(t *testing.T) {
+	testChanSendBarrier(false)
+}
+
+func testChanSendBarrier(useSelect bool) {
+	var wg sync.WaitGroup
+	outer := 100
+	inner := 100000
+	if testing.Short() {
+		outer = 10
+		inner = 1000
+	}
+	for i := 0; i < outer; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			var garbage []byte
+			for j := 0; j < inner; j++ {
+				_, err := doRequest(useSelect)
+				_, ok := err.(myError)
+				if !ok {
+					panic(1)
+				}
+				garbage = make([]byte, 1<<10)
+			}
+			global = garbage
+		}()
+	}
+	wg.Wait()
+}

src/runtime/select.go

@@ -575,7 +575,7 @@ syncsend:
 		print("syncsend: sel=", sel, " c=", c, "\n")
 	}
 	if sg.elem != nil {
-		typedmemmove(c.elemtype, sg.elem, cas.elem)
+		syncsend(c, sg, cas.elem)
 	}
 	sg.elem = nil
 	gp = sg.g