runtime: fix sleep/wakeup race for GC assists

GC assists check gcBlackenEnabled under the assist queue lock to avoid
going to sleep after gcWakeAllAssists has already woken all assists.
However, currently we clear gcBlackenEnabled shortly *after* waking
all assists, which opens a window where this exact race can happen.

Fix this by clearing gcBlackenEnabled before waking blocked assists.
However, it's unlikely this actually matters because the world is
stopped between waking assists and clearing gcBlackenEnabled and there
aren't any obvious allocations during this window, so I don't think an
assist could actually slip in to this race window.

Updates #13645.

Change-Id: I7571f059530481dc781d8fd96a1a40aadebecb0d
Reviewed-on: https://go-review.googlesource.com/18682
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgc.go

@@ -1121,6 +1121,10 @@ top:
 		// finalizers have been scanned.
 		work.finalizersDone = true
 
+		// Disable assists and background workers. We must do
+		// this before waking blocked assists.
+		atomic.Store(&gcBlackenEnabled, 0)
+
 		// Flush the gcWork caches. This must be done before
 		// endCycle since endCycle depends on statistics kept
 		// in these caches.

src/runtime/mgcmark.go

@@ -491,7 +491,8 @@ retry:
 }
 
 // gcWakeAllAssists wakes all currently blocked assists. This is used
-// at the end of a GC cycle.
+// at the end of a GC cycle. gcBlackenEnabled must be false to prevent
+// new assists from going to sleep after this point.
 func gcWakeAllAssists() {
 	lock(&work.assistQueue.lock)
 	injectglist(work.assistQueue.head.ptr())