runtime/race: os.File.ReadAt defeats race detector

[dominikh]

What version of Go are you using (go version)?

$ go version
go version devel go1.19-45f45444b3 Sat Mar 5 21:20:16 2022 +0000 linux/amd64
go version go1.17.8 linux/amd64
go version go1.15.15 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/dominikh/.cache/go-build"
GOENV="/home/dominikh/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/dominikh/prj/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/dominikh/prj"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/dominikh/prj/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/dominikh/prj/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.17.8"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/dominikh/prj/src/honnef.co/go/bittorrent/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build2412946897=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Run

package main

import "os"

func main() {
	buf := make([]byte, 1)
	f, err := os.Open("/dev/urandom")
	if err != nil {
		panic(err)
	}

	go func() {
		for {
			n, err := f.ReadAt(buf, 0)
			if err != nil || n == 0 {
				panic(err)
			}
		}
	}()

	for {
		println(buf[0])
	}
}

with GORACE=halt_on_error=1 go run -race foo.go

What did you expect to see?

I expect the race detector to flag the race.

What did you see instead?

The race detector misses the race and we get an infinite stream of random bytes.

Using Read instead of ReadAt almost immediately flags the race. The issue isn't specific to the file being opened. It happens just the same with files in /proc or actual files on a hard drive.

/cc @golang/runtime

[dominikh]

Quick investigation by @ericlagergren suggests that syscall.Pread (and Pwrite) are missing the required runtime calls, unlike Read and Write.

[ianlancetaylor]

Thanks for the analysis.

[gopherbot]

Change https://go.dev/cl/391954 mentions this issue: syscall: add race annotations to Pread and Pwrite

[gopherbot]

Change https://go.dev/cl/392774 mentions this issue: syscall: add race annotations to Windows ReadFile and WriteFile

[gopherbot]

Change https://go.dev/cl/392794 mentions this issue: unix: add race annotations to Pread and Pwrite

[gopherbot]

Change https://go.dev/cl/392814 mentions this issue: windows: add race annotations to Windows ReadFile and WriteFile