Data Race Detector 数据种类探测器:数据种类探测器手册
Introduction
Data races are among the most common and hardest to debug types of bugs in concurrent systems. A data race occurs when two goroutines access the same variable concurrently and at least one of the accesses is a write. See the The Go Memory Model for details.
Here is an example of a data race that can lead to crashes and memory corruption:
func main() { c := make(chan bool) m := make(map[string]string) go func() { m["1"] = "a" // First conflicting access. c <- true }() m["2"] = "b" // Second conflicting access. <-c for k, v := range m { fmt.Println(k, v) } }
Usage
To help diagnose such bugs, Go includes a built-in data race detector. To use it, add the -race
flag to the go command:
$ go test -race mypkg // to test the package $ go run -race mysrc.go // to run the source file $ go build -race mycmd // to build the command $ go install -race mypkg // to install the package
Report Format
When the race detector finds a data race in the program, it prints a report. The report contains stack traces for conflicting accesses, as well as stacks where the involved goroutines were created. Here is an example:
WARNING: DATA RACE Read by goroutine 185: net.(*pollServer).AddFD() src/net/fd_unix.go:89 +0x398 net.(*pollServer).WaitWrite() src/net/fd_unix.go:247 +0x45 net.(*netFD).Write() src/net/fd_unix.go:540 +0x4d4 net.(*conn).Write() src/net/net.go:129 +0x101 net.func·060() src/net/timeout_test.go:603 +0xaf Previous write by goroutine 184: net.setWriteDeadline() src/net/sockopt_posix.go:135 +0xdf net.setDeadline() src/net/sockopt_posix.go:144 +0x9c net.(*conn).SetDeadline() src/net/net.go:161 +0xe3 net.func·061() src/net/timeout_test.go:616 +0x3ed Goroutine 185 (running) created at: net.func·061() src/net/timeout_test.go:609 +0x288 Goroutine 184 (running) created at: net.TestProlongTimeout() src/net/timeout_test.go:618 +0x298 testing.tRunner() src/testing/testing.go:301 +0xe8
Options
The GORACE
environment variable sets race detector options. The format is:
GORACE="option1=val1 option2=val2"
The options are:
log_path
(defaultstderr
): The race detector writes its report to a file namedlog_path.pid
. The special namesstdout
andstderr
cause reports to be written to standard output and standard error, respectively.exitcode
(default66
): The exit status to use when exiting after a detected race.strip_path_prefix
(default""
): Strip this prefix from all reported file paths, to make reports more concise.history_size
(default1
): The per-goroutine memory access history is32K * 2**history_size elements
. Increasing this value can avoid a "failed to restore the stack" error in reports, at the cost of increased memory usage.halt_on_error
(default0
): Controls whether the program exits after reporting first data race.
Example:
$ GORACE="log_path=/tmp/race/report strip_path_prefix=/my/go/sources/" go test -race
Excluding Tests
When you build with -race
flag, the go
command defines additional build tag race
. You can use the tag to exclude some code and tests when running the race detector. Some examples:
// +build !race package foo // The test contains a data race. See issue 123. func TestFoo(t *testing.T) { // ... } // The test fails under the race detector due to timeouts. func TestBar(t *testing.T) { // ... } // The test takes too long under the race detector. func TestBaz(t *testing.T) { // ... }
How To Use
To start, run your tests using the race detector (go test -race
). The race detector only finds races that happen at runtime, so it can't find races in code paths that are not executed. If your tests have incomplete coverage, you may find more races by running a binary built with -race
under a realistic workload.
Typical Data Races
Here are some typical data races. All of them can be detected with the race detector.
Race on loop counter
func main() { var wg sync.WaitGroup wg.Add(5) for i := 0; i < 5; i++ { go func() { fmt.Println(i) // Not the 'i' you are looking for. wg.Done() }() } wg.Wait() }
The variable i
in the function literal is the same variable used by the loop, so the read in the goroutine races with the loop increment. (This program typically prints 55555, not 01234.) The program can be fixed by making a copy of the variable:
func main() { var wg sync.WaitGroup wg.Add(5) for i := 0; i < 5; i++ { go func(j int) { fmt.Println(j) // Good. Read local copy of the loop counter. wg.Done() }(i) } wg.Wait() }
Accidentally shared variable
// ParallelWrite writes data to file1 and file2, returns the errors. func ParallelWrite(data []byte) chan error { res := make(chan error, 2) f1, err := os.Create("file1") if err != nil { res <- err } else { go func() { // This err is shared with the main goroutine, // so the write races with the write below. _, err = f1.Write(data) res <- err f1.Close() }() } f2, err := os.Create("file2") // The second conflicting write to err. if err != nil { res <- err } else { go func() { _, err = f2.Write(data) res <- err f2.Close() }() } return res }
The fix is to introduce new variables in the goroutines (note the use of :=
):
... _, err := f1.Write(data) ... _, err := f2.Write(data) ...
Unprotected global variable
If the following code is called from several goroutines, it leads to races on the service
map. Concurrent reads and writes of the same map are not safe:
var service map[string]net.Addr func RegisterService(name string, addr net.Addr) { service[name] = addr } func LookupService(name string) net.Addr { return service[name] }
To make the code safe, protect the accesses with a mutex:
var ( service map[string]net.Addr serviceMu sync.Mutex ) func RegisterService(name string, addr net.Addr) { serviceMu.Lock() defer serviceMu.Unlock() service[name] = addr } func LookupService(name string) net.Addr { serviceMu.Lock() defer serviceMu.Unlock() return service[name] }
Primitive unprotected variable
Data races can happen on variables of primitive types as well (bool
, int
, int64
, etc.), as in this example:
type Watchdog struct{ last int64 } func (w *Watchdog) KeepAlive() { w.last = time.Now().UnixNano() // First conflicting access. } func (w *Watchdog) Start() { go func() { for { time.Sleep(time.Second) // Second conflicting access. if w.last < time.Now().Add(-10*time.Second).UnixNano() { fmt.Println("No keepalives for 10 seconds. Dying.") os.Exit(1) } } }() }
Even such "innocent" data races can lead to hard-to-debug problems caused by non-atomicity of the memory accesses, interference with compiler optimizations, or reordering issues accessing processor memory .
A typical fix for this race is to use a channel or a mutex. To preserve the lock-free behavior, one can also use thesync/atomic
package.
type Watchdog struct{ last int64 } func (w *Watchdog) KeepAlive() { atomic.StoreInt64(&w.last, time.Now().UnixNano()) } func (w *Watchdog) Start() { go func() { for { time.Sleep(time.Second) if atomic.LoadInt64(&w.last) < time.Now().Add(-10*time.Second).UnixNano() { fmt.Println("No keepalives for 10 seconds. Dying.") os.Exit(1) } } }() }
Supported Systems
The race detector runs on darwin/amd64
, freebsd/amd64
, linux/amd64
, and windows/amd64
.
Runtime Overhead
The cost of race detection varies by program, but for a typical program, memory usage may increase by 5-10x and execution time by 2-20x.