vfs: fix very high load caused by slow directory listings

In this commit (released in v1.52.0)

6ca7198f mount: fix disappearing cwd problem

SetSys was introduced to cache node lookups.

Unfortunately taking the vfs.(*Dir) lock in SetSys causes any FUSE
operations on a directory to pile up behind slow directory listings.

In some situations this leads to very high load.

This commit fixes it by using atomic operations to read and write the
Sys value make it independent of the lock.

See: https://forum.rclone.org/t/high-cpu-load-with-rclone-mount/17604
See: #4104

vfs/dir.go

@@ -7,6 +7,7 @@ import (
 	"sort"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/pkg/errors"
@@ -33,7 +34,7 @@ type Dir struct {
 	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     interface{}       // user defined info to be attached here
+	sys     atomic.Value      // user defined info to be attached here
 }
 
 //go:generate stringer -type=vState
@@ -105,16 +106,12 @@ func (d *Dir) Path() (name string) {
 
 // Sys returns underlying data source (can be nil) - satisfies Node interface
 func (d *Dir) Sys() interface{} {
-	d.mu.RLock()
-	defer d.mu.RUnlock()
-	return d.sys
+	return d.sys.Load()
 }
 
 // SetSys sets the underlying data source (can be nil) - satisfies Node interface
 func (d *Dir) SetSys(x interface{}) {
-	d.mu.Lock()
-	d.sys = x
-	d.mu.Unlock()
+	d.sys.Store(x)
 }
 
 // Inode returns the inode number - satisfies Node interface

vfs/file.go

@@ -48,7 +48,7 @@ type File struct {
 	pendingModTime   time.Time                       // will be applied once o becomes available, i.e. after file was written
 	pendingRenameFun func(ctx context.Context) error // will be run/renamed after all writers close
 	appendMode       bool                            // file was opened with O_APPEND
-	sys              interface{}                     // user defined info to be attached here
+	sys              atomic.Value                    // user defined info to be attached here
 
 	muRW sync.Mutex // synchronize RWFileHandle.openPending(), RWFileHandle.close() and File.Remove
 }
@@ -122,16 +122,12 @@ func (f *File) Path() string {
 
 // Sys returns underlying data source (can be nil) - satisfies Node interface
 func (f *File) Sys() interface{} {
-	f.mu.RLock()
-	defer f.mu.RUnlock()
-	return f.sys
+	return f.sys.Load()
 }
 
 // SetSys sets the underlying data source (can be nil) - satisfies Node interface
 func (f *File) SetSys(x interface{}) {
-	f.mu.Lock()
-	f.sys = x
-	f.mu.Unlock()
+	f.sys.Store(x)
 }
 
 // Inode returns the inode number - satisfies Node interface