os: fix race condition in readdir by atomically initializing dirinfo

This change ensures that dirinfo in the File struct is initialized atomically,
avoiding redundant allocations when multiple goroutines access it concurrently.
Instead of creating separate buffers, we now use CompareAndSwap to guarantee
thread-safe initialization and reduce unnecessary memory usage.

Although this is not a strict race condition, the update enhances efficiency by
eliminating duplicate allocations and ensuring safer concurrent access.

Fixes #71496.

Change-Id: If08699a94afa05611cdf67e82a5957a8d8f9d5c8
GitHub-Last-Rev: 1e1f6191439cf3ad32f3ba54bba5a0185dd55b14
GitHub-Pull-Request: golang/go#71501
Reviewed-on: https://go-review.googlesource.com/c/go/+/645720
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

diff --git a/src/os/dir_plan9.go b/src/os/dir_plan9.go
index ab5c1efce5b075294c6964d64f812779b235ff4c..9d28bd7dda4a890ca05ac6b66552d49e9919a8ae 100644 (file)
--- a/src/os/dir_plan9.go
+++ b/src/os/dir_plan9.go
@@ -11,12 +11,19 @@ import (
 )
 
 func (file *File) readdir(n int, mode readdirMode) (names []string, dirents []DirEntry, infos []FileInfo, err error) {
-       // If this file has no dirinfo, create one.
-       d := file.dirinfo.Load()
-       if d == nil {
-               d = new(dirInfo)
-               file.dirinfo.Store(d)
+       var d *dirInfo
+       for {
+               d = file.dirinfo.Load()
+               if d != nil {
+                       break
+               }
+               newD := new(dirInfo)
+               if file.dirinfo.CompareAndSwap(nil, newD) {
+                       d = newD
+                       break
+               }
        }
+
        d.mu.Lock()
        defer d.mu.Unlock()
 

diff --git a/src/os/dir_unix.go b/src/os/dir_unix.go
index eadc1660c212eabca5e26ae6ae3d887190811b23..6a0135b70b073b2483e0af30a4e92a14ffe2ab09 100644 (file)
--- a/src/os/dir_unix.go
+++ b/src/os/dir_unix.go
@@ -46,11 +46,19 @@ func (d *dirInfo) close() {
 
 func (f *File) readdir(n int, mode readdirMode) (names []string, dirents []DirEntry, infos []FileInfo, err error) {
        // If this file has no dirInfo, create one.
-       d := f.dirinfo.Load()
-       if d == nil {
-               d = new(dirInfo)
-               f.dirinfo.Store(d)
+       var d *dirInfo
+       for {
+               d = f.dirinfo.Load()
+               if d != nil {
+                       break
+               }
+               newD := new(dirInfo)
+               if f.dirinfo.CompareAndSwap(nil, newD) {
+                       d = newD
+                       break
+               }
        }
+
        d.mu.Lock()
        defer d.mu.Unlock()
        if d.buf == nil {