math/rand: use of Read is racy

[yonderblue]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   1.7rc1
2. What operating system and processor architecture are you using (go env)?
   linux amd64
3. What did you do?
   Ran a large concurrent unit test.
4. What did you expect to see?
   Bunnies
5. What did you see instead?

WARNING: DATA RACE
Write at 0x00c420098098 by goroutine 65:
  math/rand.(*Rand).Seed()
      /usr/local/go/src/math/rand/rand.go:54 +0x6d
  math/rand.Seed()
      /usr/local/go/src/math/rand/rand.go:202 +0x54
  XXX/XXX_test.TestXXX.func2()
      /go/src/XXX/XXX_test.go:359 +0x163

Previous write at 0x00c420098098 by goroutine 66:
  math/rand.(*Rand).Seed()
      /usr/local/go/src/math/rand/rand.go:54 +0x6d
  math/rand.Seed()
      /usr/local/go/src/math/rand/rand.go:202 +0x54
  XXX/XXX_test.TestXXX.func2()
      /go/src/XXX/XXX_test.go:359 +0x163

Goroutine 65 (running) created at:
  XXX/XXX_test.TestXXX()
      /go/src/XXX/XXX_test.go:389 +0x8c2
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:610 +0xc9

Goroutine 66 (running) created at:
  XXX/XXX_test.TestXXX()
      /go/src/XXX/XXX_test.go:389 +0x8c2
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:610 +0xc9
==================
==================
WARNING: DATA RACE
Read at 0x00c420098090 by goroutine 65:
  math/rand.(*Rand).Read()
      /usr/local/go/src/math/rand/rand.go:177 +0x6a
  math/rand.Read()
      /usr/local/go/src/math/rand/rand.go:248 +0x6c
  XXX/XXX_test.TestXXX.func2()
      /go/src/XXX/XXX_test.go:360 +0x188

Previous write at 0x00c420098090 by goroutine 66:
  math/rand.(*Rand).Read()
      /usr/local/go/src/math/rand/rand.go:188 +0x127
  math/rand.Read()
      /usr/local/go/src/math/rand/rand.go:248 +0x6c
  XXX/XXX_test.TestXXX.func2()
      /go/src/XXX/XXX_test.go:360 +0x188

Goroutine 65 (running) created at:
  XXX/XXX_test.TestXXX()
      /go/src/XXX/XXX_test.go:389 +0x8c2
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:610 +0xc9

Goroutine 66 (running) created at:
  XXX/XXX_test.TestXXX()
      /go/src/XXX/XXX_test.go:389 +0x8c2
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:610 +0xc9
==================

I can confirm it does not happen on 1.6, at least with my runs.

[dsnet]

/cc @adg @ianlancetaylor @randall77

The recent commit 8d966ba for #16124 has no locks over the readPos and readVal fields. The documentation for math/rand seems to indicate that concurrent calls of Read should be permitted.

We could use a sync.Mutex or atomic instructions to update those fields.

[gopherbot]

CL https://golang.org/cl/24839 mentions this issue.

[gopherbot]

CL https://golang.org/cl/24852 mentions this issue.

[gopherbot]

CL https://golang.org/cl/24857 mentions this issue.