runtime: document non-obvious requirement on sudog.elem

The channel code must not allow stack splits between when it assigns a
potential stack pointer to sudog.elem (or sudog.selectdone) and when
it makes the sudog visible to copystack by putting it on the g.waiting
list. We do get this right everywhere, but add a comment about this
subtlety for future eyes.

Change-Id: I941da150437167acff37b0e56983c793f40fcf79
Reviewed-on: https://go-review.googlesource.com/19632Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/chan.go

@@ -203,6 +203,8 @@ func chansend(t *chantype, c *hchan, ep unsafe.Pointer, block bool, callerpc uin
 	if t0 != 0 {
 		mysg.releasetime = -1
 	}
+	// No stack splits between assigning elem and enqueuing mysg
+	// on gp.waiting where copystack can find it.
 	mysg.elem = ep
 	mysg.waitlink = nil
 	mysg.g = gp
@@ -460,6 +462,8 @@ func chanrecv(t *chantype, c *hchan, ep unsafe.Pointer, block bool) (selected, r
 	if t0 != 0 {
 		mysg.releasetime = -1
 	}
+	// No stack splits between assigning elem and enqueuing mysg
+	// on gp.waiting where copystack can find it.
 	mysg.elem = ep
 	mysg.waitlink = nil
 	gp.waiting = mysg

src/runtime/select.go

@@ -370,6 +370,8 @@ loop:
 		sg.g = gp
 		// Note: selectdone is adjusted for stack copies in stack1.go:adjustsudogs
 		sg.selectdone = (*uint32)(noescape(unsafe.Pointer(&done)))
+		// No stack splits between assigning elem and enqueuing
+		// sg on gp.waiting where copystack can find it.
 		sg.elem = cas.elem
 		sg.releasetime = 0
 		if t0 != 0 {