• 28 Data Race Detector 数据种类探测器:数据种类探测器手册



    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 (default stderr): The race detector writes its report to a file named log_path.pid. The special names stdout and stderr cause reports to be written to standard output and standard error, respectively.
    • exitcode (default 66): 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 (default 1): The per-goroutine memory access history is 32K * 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 (default 0): 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 (boolintint64, 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/amd64freebsd/amd64linux/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.

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  • 原文地址:https://www.cnblogs.com/kaid/p/9698551.html
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