sync: throw, not panic, for unlock of unlocked mutex

The panic leaves the lock in an unusable state.
Trying to panic with a usable state makes the lock significantly
less efficient and scalable (see early CL patch sets and discussion).

Instead, use runtime.throw, which will crash the program directly.

In general throw is reserved for when the runtime detects truly
serious, unrecoverable problems. This problem is certainly serious,
and, without a significant performance hit, is unrecoverable.

Fixes #13879.

Change-Id: I41920d9e2317270c6f909957d195bd8b68177f8d
Reviewed-on: https://go-review.googlesource.com/31359Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/panic.go

@@ -576,6 +576,11 @@ func dopanic(unused int) {
 	*(*int)(nil) = 0
 }
 
+//go:linkname sync_throw sync.throw
+func sync_throw(s string) {
+	throw(s)
+}
+
 //go:nosplit
 func throw(s string) {
 	print("fatal error: ", s, "\n")

src/sync/mutex.go

@@ -16,6 +16,8 @@ import (
 	"unsafe"
 )
 
+func throw(string) // provided by runtime
+
 // A Mutex is a mutual exclusion lock.
 // Mutexes can be created as part of other structures;
 // the zero value for a Mutex is an unlocked mutex.
@@ -74,7 +76,7 @@ func (m *Mutex) Lock() {
 			// The goroutine has been woken from sleep,
 			// so we need to reset the flag in either case.
 			if new&mutexWoken == 0 {
-				panic("sync: inconsistent mutex state")
+				throw("sync: inconsistent mutex state")
 			}
 			new &^= mutexWoken
 		}
@@ -108,7 +110,7 @@ func (m *Mutex) Unlock() {
 	// Fast path: drop lock bit.
 	new := atomic.AddInt32(&m.state, -mutexLocked)
 	if (new+mutexLocked)&mutexLocked == 0 {
-		panic("sync: unlock of unlocked mutex")
+		throw("sync: unlock of unlocked mutex")
 	}
 
 	old := new

src/sync/mutex_test.go

@@ -7,7 +7,12 @@
 package sync_test
 
 import (
+	"fmt"
+	"internal/testenv"
+	"os"
+	"os/exec"
 	"runtime"
+	"strings"
 	. "sync"
 	"testing"
 )
@@ -71,17 +76,98 @@ func TestMutex(t *testing.T) {
 	}
 }
 
-func TestMutexPanic(t *testing.T) {
-	defer func() {
-		if recover() == nil {
-			t.Fatalf("unlock of unlocked mutex did not panic")
+var misuseTests = []struct {
+	name string
+	f    func()
+}{
+	{
+		"Mutex.Unlock",
+		func() {
+			var mu Mutex
+			mu.Unlock()
+		},
+	},
+	{
+		"Mutex.Unlock2",
+		func() {
+			var mu Mutex
+			mu.Lock()
+			mu.Unlock()
+			mu.Unlock()
+		},
+	},
+	{
+		"RWMutex.Unlock",
+		func() {
+			var mu RWMutex
+			mu.Unlock()
+		},
+	},
+	{
+		"RWMutex.Unlock2",
+		func() {
+			var mu RWMutex
+			mu.RLock()
+			mu.Unlock()
+		},
+	},
+	{
+		"RWMutex.Unlock3",
+		func() {
+			var mu RWMutex
+			mu.Lock()
+			mu.Unlock()
+			mu.Unlock()
+		},
+	},
+	{
+		"RWMutex.RUnlock",
+		func() {
+			var mu RWMutex
+			mu.RUnlock()
+		},
+	},
+	{
+		"RWMutex.RUnlock2",
+		func() {
+			var mu RWMutex
+			mu.Lock()
+			mu.RUnlock()
+		},
+	},
+	{
+		"RWMutex.RUnlock3",
+		func() {
+			var mu RWMutex
+			mu.RLock()
+			mu.RUnlock()
+			mu.RUnlock()
+		},
+	},
+}
+
+func init() {
+	if len(os.Args) == 3 && os.Args[1] == "TESTMISUSE" {
+		for _, test := range misuseTests {
+			if test.name == os.Args[2] {
+				test.f()
+				fmt.Printf("test completed\n")
+				os.Exit(0)
+			}
 		}
-	}()
+		fmt.Printf("unknown test\n")
+		os.Exit(0)
+	}
+}
 
-	var mu Mutex
-	mu.Lock()
-	mu.Unlock()
-	mu.Unlock()
+func TestMutexMisuse(t *testing.T) {
+	testenv.MustHaveExec(t)
+	for _, test := range misuseTests {
+		out, err := exec.Command(os.Args[0], "TESTMISUSE", test.name).CombinedOutput()
+		if err == nil || !strings.Contains(string(out), "unlocked") {
+			t.Errorf("%s: did not find failure with message about unlocked lock: %s\n%s\n", test.name, err, out)
+		}
+	}
 }
 
 func BenchmarkMutexUncontended(b *testing.B) {

src/sync/rwmutex.go

@@ -61,7 +61,7 @@ func (rw *RWMutex) RUnlock() {
 	if r := atomic.AddInt32(&rw.readerCount, -1); r < 0 {
 		if r+1 == 0 || r+1 == -rwmutexMaxReaders {
 			race.Enable()
-			panic("sync: RUnlock of unlocked RWMutex")
+			throw("sync: RUnlock of unlocked RWMutex")
 		}
 		// A writer is pending.
 		if atomic.AddInt32(&rw.readerWait, -1) == 0 {
@@ -115,7 +115,7 @@ func (rw *RWMutex) Unlock() {
 	r := atomic.AddInt32(&rw.readerCount, rwmutexMaxReaders)
 	if r >= rwmutexMaxReaders {
 		race.Enable()
-		panic("sync: Unlock of unlocked RWMutex")
+		throw("sync: Unlock of unlocked RWMutex")
 	}
 	// Unblock blocked readers, if any.
 	for i := 0; i < int(r); i++ {

src/sync/rwmutex_test.go

@@ -155,48 +155,6 @@ 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