runtime: make gcFlushBgCredit go:nowritebarrierrec

Write barriers in gcFlushBgCredit lead to very subtle bugs because it
executes after the getfull barrier. I tracked some bugs of this form
down before go:nowritebarrierrec was implemented. Ensure that they
don't reappear by making gcFlushBgCredit go:nowritebarrierrec.

Change-Id: Ia5ca2dc59e6268bce8d8b4c87055bd0f6e19bed2
Reviewed-on: https://go-review.googlesource.com/17052Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/mgcmark.go

@@ -504,6 +504,12 @@ func gcWakeAllAssists() {
 // credit. This first satisfies blocked assists on the
 // work.assistQueue and then flushes any remaining credit to
 // gcController.bgScanCredit.
+//
+// Write barriers are disallowed because this is used by gcDrain after
+// it has ensured that all work is drained and this must preserve that
+// condition.
+//
+//go:nowritebarrierrec
 func gcFlushBgCredit(scanWork int64) {
 	if work.assistQueue.head == 0 {
 		// Fast path; there are no blocked assists. There's a
@@ -846,6 +852,10 @@ func gcDrain(gcw *gcWork, flags gcDrainFlags) {
 		}
 	}
 
+	// In blocking mode, write barriers are not allowed after this
+	// point because we must preserve the condition that the work
+	// buffers are empty.
+
 	// Flush remaining scan work credit.
 	if gcw.scanWork > 0 {
 		atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)