sync: detect incorrect usages of RWMutex

Fixes #7858.

LGTM=ruiu
R=ruiu
CC=golang-codereviews
https://golang.org/cl/92720045

src/pkg/sync/rwmutex.go

@@ -51,7 +51,11 @@ func (rw *RWMutex) RUnlock() {
 		raceReleaseMerge(unsafe.Pointer(&rw.writerSem))
 		raceDisable()
 	}
-	if atomic.AddInt32(&rw.readerCount, -1) < 0 {
+	if r := atomic.AddInt32(&rw.readerCount, -1); r < 0 {
+		if r+1 == 0 || r+1 == -rwmutexMaxReaders {
+			raceEnable()
+			panic("sync: RUnlock of unlocked RWMutex")
+		}
 		// A writer is pending.
 		if atomic.AddInt32(&rw.readerWait, -1) == 0 {
 			// The last reader unblocks the writer.
@@ -105,6 +109,10 @@ func (rw *RWMutex) Unlock() {
 
 	// Announce to readers there is no active writer.
 	r := atomic.AddInt32(&rw.readerCount, rwmutexMaxReaders)
+	if r >= rwmutexMaxReaders {
+		raceEnable()
+		panic("sync: Unlock of unlocked RWMutex")
+	}
 	// Unblock blocked readers, if any.
 	for i := 0; i < int(r); i++ {
 		runtime_Semrelease(&rw.readerSem)

src/pkg/sync/rwmutex_test.go

@@ -155,6 +155,48 @@ func TestRLocker(t *testing.T) {
 	}
 }
 
+func TestUnlockPanic(t *testing.T) {
+	defer func() {
+		if recover() == nil {
+			t.Fatalf("unlock of unlocked RWMutex did not panic")
+		}
+	}()
+	var mu RWMutex
+	mu.Unlock()
+}
+
+func TestUnlockPanic2(t *testing.T) {
+	defer func() {
+		if recover() == nil {
+			t.Fatalf("unlock of unlocked RWMutex did not panic")
+		}
+	}()
+	var mu RWMutex
+	mu.RLock()
+	mu.Unlock()
+}
+
+func TestRUnlockPanic(t *testing.T) {
+	defer func() {
+		if recover() == nil {
+			t.Fatalf("read unlock of unlocked RWMutex did not panic")
+		}
+	}()
+	var mu RWMutex
+	mu.RUnlock()
+}
+
+func TestRUnlockPanic2(t *testing.T) {
+	defer func() {
+		if recover() == nil {
+			t.Fatalf("read unlock of unlocked RWMutex did not panic")
+		}
+	}()
+	var mu RWMutex
+	mu.Lock()
+	mu.RUnlock()
+}
+
 func BenchmarkRWMutexUncontended(b *testing.B) {
 	type PaddedRWMutex struct {
 		RWMutex