[release-branch.go1.7] runtime: improve diagnostics for "scan missed a g"

Updates #18700 (backport)

Currently there are no diagnostics for mark root check during marking.
Fix this by printing out the same diagnostics we print during mark
termination.

Also, drop the allglock before throwing. Holding that across a throw
causes a self-deadlock with tracebackothers.

For #16083.

Change-Id: Ib605f3ae0c17e70704b31d8378274cfaa2307dc2
Reviewed-on: https://go-review.googlesource.com/35677
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/mgcmark.go

@@ -106,26 +106,32 @@ func gcMarkRootCheck() {
 
 	lock(&allglock)
 	// Check that stacks have been scanned.
+	var gp *g
 	if gcphase == _GCmarktermination {
 		for i := 0; i < len(allgs); i++ {
-			gp := allgs[i]
+			gp = allgs[i]
 			if !(gp.gcscandone && gp.gcscanvalid) && readgstatus(gp) != _Gdead {
-				println("gp", gp, "goid", gp.goid,
-					"status", readgstatus(gp),
-					"gcscandone", gp.gcscandone,
-					"gcscanvalid", gp.gcscanvalid)
-				throw("scan missed a g")
+				goto fail
 			}
 		}
 	} else {
 		for i := 0; i < work.nStackRoots; i++ {
-			gp := allgs[i]
+			gp = allgs[i]
 			if !gp.gcscandone {
-				throw("scan missed a g")
+				goto fail
 			}
 		}
 	}
 	unlock(&allglock)
+	return
+
+fail:
+	println("gp", gp, "goid", gp.goid,
+		"status", readgstatus(gp),
+		"gcscandone", gp.gcscandone,
+		"gcscanvalid", gp.gcscanvalid)
+	unlock(&allglock) // Avoid self-deadlock with traceback.
+	throw("scan missed a g")
 }
 
 // ptrmask for an allocation containing a single pointer.