lockdep: fix the TOCTTOU issue

The problem is that it checkes circular dependencies and one then adds the
target lock to the graph.

Reported-by: syzbot+7687e27a2029723c7fb4@syzkaller.appspotmail.com
PiperOrigin-RevId: 512637945

pkg/sync/locking/lockdep.go

@@ -72,9 +72,23 @@ type goroutineLocks map[*MutexClass]bool
 
 var routineLocks goroutineLocksAtomicPtrMap
 
+// maxChainLen is the maximum length of a lock chain.
+const maxChainLen = 32
+
 // checkLock checks that class isn't in the ancestors of prevClass.
 func checkLock(class *MutexClass, prevClass *MutexClass, chain []*MutexClass) {
 	chain = append(chain, prevClass)
+	if len(chain) >= maxChainLen {
+		// It can be a race condition with another thread that added
+		// the lock to the graph but don't complete the validation.
+		var b strings.Builder
+		fmt.Fprintf(&b, "WARNING: The maximum lock depth has been reached: %s", chain[0])
+		for i := 1; i < len(chain); i++ {
+			fmt.Fprintf(&b, "-> %s", chain[i])
+		}
+		log.Warningf("%s", b.String())
+		return
+	}
 	if c := prevClass.ancestors.Load(class); c != nil {
 		var b strings.Builder
 		fmt.Fprintf(&b, "WARNING: circular locking detected: %s -> %s:\n%s\n",
@@ -118,10 +132,14 @@ func AddGLock(class *MutexClass, lockNameIndex int) {
 		return
 	}
 
+	if (*currentLocks)[class] {
+		panic(fmt.Sprintf("nested locking: %s:\n%s", class, log.LocalStack(2)))
+	}
+	(*currentLocks)[class] = true
 	// Check dependencies and add locked mutexes to the ancestors list.
 	for prevClass := range *currentLocks {
 		if prevClass == class {
-			panic(fmt.Sprintf("nested locking: %s:\n%s", class, log.LocalStack(2)))
+			continue
 		}
 		checkLock(class, prevClass, nil)
 
@@ -130,7 +148,6 @@ func AddGLock(class *MutexClass, lockNameIndex int) {
 			class.ancestors.Store(prevClass, &stacks)
 		}
 	}
-	(*currentLocks)[class] = true
 }
 
 // DelGLock deletes a lock from the current goroutine.