vfs: fix directory locking caused by slow directory listings

Before this fix we took the directory lock to read the ModTime of the
directory. This was causing locking on directories which were being
re-read from the backend.

This commit gives the modtime its own lock so it can be read even when
the directory is being updated.

See: https://forum.rclone.org/t/high-cpu-load-with-rclone-mount/17604

vfs/dir.go

@@ -29,12 +29,14 @@ type Dir struct {
 	mu      sync.RWMutex // protects the following
 	parent  *Dir         // parent, nil for root
 	path    string
-	modTime time.Time
 	entry   fs.Directory
 	read    time.Time         // time directory entry last read
 	items   map[string]Node   // directory entries - can be empty but not nil
 	virtual map[string]vState // virtual directory entries - may be nil
 	sys     atomic.Value      // user defined info to be attached here
+
+	modTimeMu sync.Mutex // protects the following
+	modTime   time.Time
 }
 
 //go:generate stringer -type=vState
@@ -270,11 +272,13 @@ func (d *Dir) _age(when time.Time) (age time.Duration, stale bool) {
 // reading everything again
 func (d *Dir) rename(newParent *Dir, fsDir fs.Directory) {
 	d.ForgetAll()
+	d.modTimeMu.Lock()
+	d.modTime = fsDir.ModTime(context.TODO())
+	d.modTimeMu.Unlock()
 	d.mu.Lock()
 	d.parent = newParent
 	d.entry = fsDir
 	d.path = fsDir.Remote()
-	d.modTime = fsDir.ModTime(context.TODO())
 	d.read = time.Time{}
 	d.mu.Unlock()
 }
@@ -550,8 +554,8 @@ func (d *Dir) isEmpty() (bool, error) {
 
 // ModTime returns the modification time of the directory
 func (d *Dir) ModTime() time.Time {
-	d.mu.RLock()
-	defer d.mu.RUnlock()
+	d.modTimeMu.Lock()
+	defer d.modTimeMu.Unlock()
 	// fs.Debugf(d.path, "Dir.ModTime %v", d.modTime)
 	return d.modTime
 }
@@ -566,9 +570,9 @@ func (d *Dir) SetModTime(modTime time.Time) error {
 	if d.vfs.Opt.ReadOnly {
 		return EROFS
 	}
-	d.mu.Lock()
+	d.modTimeMu.Lock()
 	d.modTime = modTime
-	d.mu.Unlock()
+	d.modTimeMu.Unlock()
 	return nil
 }