diff --git a/samples/cachingfs/caching_fs.go b/samples/cachingfs/caching_fs.go
index ffe1d61..7c48931 100644
--- a/samples/cachingfs/caching_fs.go
+++ b/samples/cachingfs/caching_fs.go
@@ -15,14 +15,20 @@
 package cachingfs
 
 import (
-	"sync"
 	"time"
 
 	"github.com/jacobsa/fuse"
 	"github.com/jacobsa/fuse/fuseutil"
+	"github.com/jacobsa/gcloud/syncutil"
 	"golang.org/x/net/context"
 )
 
+const (
+	// Sizes of the files according to the file system.
+	FooSize = 123
+	BarSize = 456
+)
+
 // A file system with a fixed structure that looks like this:
 //
 //     foo
@@ -64,32 +70,60 @@ type CachingFS interface {
 func NewCachingFS(
 	lookupEntryTimeout time.Duration,
 	getattrTimeout time.Duration) (fs CachingFS, err error) {
-	fs = &cachingFS{
-		inodeIDBase: fuse.RootInodeID + 1,
-		mtime:       time.Now(),
+	cfs := &cachingFS{
+		fooID: ((fuse.RootInodeID + 1 + numInodes - 1) / numInodes) + fooOffset,
+		mtime: time.Now(),
 	}
 
+	cfs.mu = syncutil.NewInvariantMutex(cfs.checkInvariants)
+
+	fs = cfs
 	return
 }
 
+const (
+	// Inode IDs are issued such that "foo" always receives an ID that is
+	// congruent to fooOffset modulo numInodes, etc.
+	fooOffset = iota
+	dirOffset
+	barOffset
+	numInodes
+)
+
 type cachingFS struct {
 	fuseutil.NotImplementedFileSystem
-	mu sync.Mutex
+	mu syncutil.InvariantMutex
 
+	// The current ID of foo.
+	//
+	// INVARIANT: FooID() > fuse.RootInodeID
+	// INVARIANT: DirID() > fuse.RootInodeID
+	// INVARIANT: BarID() > fuse.RootInodeID
+	//
+	// INVARIANT: FooID() % numInodes == fooOffset
+	// INVARIANT: DirID() % numInodes == dirOffset
+	// INVARIANT: BarID() % numInodes == barOffset
+	//
 	// GUARDED_BY(mu)
-	inodeIDBase fuse.InodeID
+	fooID fuse.InodeID
 
 	// GUARDED_BY(mu)
 	mtime time.Time
 }
 
-// LOCKS_EXCLUDED(fs.mu)
-func (fs *cachingFS) RenumberInodes(base fuse.InodeID) {
-	fs.mu.Lock()
-	defer fs.mu.Unlock()
+func (fs *cachingFS) checkInvariants()
 
-	fs.inodeIDBase = base
-}
+// LOCKS_EXCLUDED(fs.mu)
+func (fs *cachingFS) FooID() fuse.InodeID
+
+// LOCKS_EXCLUDED(fs.mu)
+func (fs *cachingFS) DirID() fuse.InodeID
+
+// LOCKS_EXCLUDED(fs.mu)
+func (fs *cachingFS) BarID() fuse.InodeID
+
+// LOCKS_EXCLUDED(fs.mu)
+func (fs *cachingFS) RenumberInodes()
 
 // LOCKS_EXCLUDED(fs.mu)
 func (fs *cachingFS) SetMtime(mtime time.Time) {