runtime: fix accounting race in printlock

It could happen that mp.printlock++ happens, then on entry to lock,
the goroutine is preempted and then rescheduled onto another m
for the actual call to lock. Now the lock and the printlock++ have
happened on different m's. This can lead to printlock not being
unlocked, which either gives a printing deadlock or a crash when
the goroutine reschedules, because m.locks > 0.

Change-Id: Ib0c08740e1b53de3a93f7ebf9b05f3dceff48b9f
Reviewed-on: https://go-review.googlesource.com/2819
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/print1.go

@@ -53,10 +53,12 @@ var debuglock mutex
 
 func printlock() {
 	mp := getg().m
+	mp.locks++ // do not reschedule between printlock++ and lock(&debuglock).
 	mp.printlock++
 	if mp.printlock == 1 {
 		lock(&debuglock)
 	}
+	mp.locks-- // now we know debuglock is held and holding up mp.locks for us.
 }
 
 func printunlock() {