DATA RACE in the TestCPUValue

[hawkingrei]

Bug Report

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1. Minimal reproduce step (Required)

==================
WARNING: DATA RACE
Read at 0x00c0002615d0 by goroutine 21:
  github.com/pingcap/tidb/util/cpu.(*Observer).observe()
      util/cpu/cpu.go:94 +0x364
  github.com/pingcap/tidb/util/cpu.(*Observer).Start.func1()
      util/cpu/cpu.go:69 +0x23a
Previous write at 0x00c0002615d0 by goroutine 10:
  github.com/pingcap/tidb/util/cpu.(*Observer).observe()
      util/cpu/cpu.go:99 +0x3bc
  github.com/pingcap/tidb/util/cpu.(*Observer).Start.func1()
      util/cpu/cpu.go:69 +0x23a
Goroutine 21 (running) created at:
  github.com/pingcap/tidb/util/cpu.(*Observer).Start()
      util/cpu/cpu.go:60 +0xa6
  github.com/pingcap/tidb/util/cpu.TestCPUValue()
      util/cpu/cpu_test.go:45 +0x19b
  testing.tRunner()
      GOROOT/src/testing/testing.go:1446 +0x216
  testing.(*T).Run.func1()
      GOROOT/src/testing/testing.go:1493 +0x47
Goroutine 10 (running) created at:
  github.com/pingcap/tidb/util/cpu.(*Observer).Start()
      util/cpu/cpu.go:60 +0xa6
  github.com/pingcap/tidb/util/cpu.TestCPUValue()
      util/cpu/cpu_test.go:28 +0x4e
  testing.tRunner()
      GOROOT/src/testing/testing.go:1446 +0x216
  testing.(*T).Run.func1()
      GOROOT/src/testing/testing.go:1493 +0x47
================== 

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)