尽管有锁，Go 仍检测到 map 上的并发读写

Question

我正在编写一个简单的缓存机制，它有一个Add、一个Evict 和一个Search 方法。 Search 目前尚未实现，因此无需担心。

调用 Add 添加数据的 goroutine 数量较多，只有一个 goroutine 运行在 evict 循环中逐出数据。一旦我对它施加一些严重的流量，Go 就会抛出并说在 map metricCache 上有并发的读写访问，但我看不出这是怎么发生的，因为它周围有锁。我正在使用 Go 1.7。

文件mdata/cache.go:

57: func NewCCache() *CCache {
58:     cc := &CCache{
59:         lock:        sync.RWMutex{},
60:         metricCache: make(map[string]*CCacheMetric),
61:         accnt:       accnt.NewFlatAccnt(maxSize),
62:     }
63:     go cc.evictLoop()
64:     return cc
65: }
66:
67: func (c *CCache) evictLoop() {
68:     evictQ := c.accnt.GetEvictQ()
69:     for target := range evictQ {
70:         c.evict(target)
71:     }
72: }
73: 
74: func (c *CCache) Add(metric string, prev uint32, itergen chunk.IterGen) {
75:     c.lock.Lock()
76: 
77:     if ccm, ok := c.metricCache[metric]; !ok {
78:         var ccm *CCacheMetric
79:         ccm = NewCCacheMetric()
80:         ccm.Init(prev, itergen)
81:         c.metricCache[metric] = ccm
82:     } else {
83:         ccm.Add(prev, itergen)
84:     }
85:     c.lock.Unlock()
86: 
87:     c.accnt.AddChunk(metric, itergen.Ts(), itergen.Size())
88: }
89: 
90: func (c *CCache) evict(target *accnt.EvictTarget) {
91:     c.lock.Lock()
92: 
93:     if _, ok := c.metricCache[target.Metric]; ok {
94:         log.Debug("cache: evicting chunk %d on metric %s\n", target.Ts, target.Metric)
95:         length := c.metricCache[target.Metric].Del(target.Ts)
96:         if length == 0 {
97:             delete(c.metricCache, target.Metric)
98:         }
99:     }
100: 
101:     c.lock.Unlock()
102: }

这是错误信息:

metrictank_1    | fatal error: concurrent map read and map write
metrictank_1    | 
metrictank_1    | goroutine 3159 [running]:
metrictank_1    | runtime.throw(0xaade7e, 0x21)
metrictank_1    |       /usr/local/go/src/runtime/panic.go:566 +0x95 fp=0xc4216a7eb8 sp=0xc4216a7e98
metrictank_1    | runtime.mapaccess2_faststr(0x9e22c0, 0xc42031e600, 0xc4210c2b10, 0x22, 0x28, 0xa585d5496)
metrictank_1    |       /usr/local/go/src/runtime/hashmap_fast.go:306 +0x52b fp=0xc4216a7f18 sp=0xc4216a7eb8
metrictank_1    | github.com/raintank/metrictank/mdata/cache.(*CCache).Add(0xc4202fa070, 0xc4210c2b10, 0x22, 0x0, 0xc421875f82, 0x25, 0x25, 0xa585d5496)
metrictank_1    |       /home/mst/go/src/github.com/raintank/metrictank/mdata/cache/cache.go:77 +0x63 fp=0xc4216a7f80 sp=0xc4216a7f18
metrictank_1    | runtime.goexit()
metrictank_1    |       /usr/local/go/src/runtime/asm_amd64.s:2086 +0x1 fp=0xc4216a7f88 sp=0xc4216a7f80
metrictank_1    | created by github.com/raintank/metrictank/api.(*Server).getSeries
metrictank_1    |       /home/mst/go/src/github.com/raintank/metrictank/api/dataprocessor.go:442 +0x122b

更新:我用 -race 重新编译，现在我得到了一个不同的错误。这看起来好像 RWMutex 完全无效，因为根据回溯，问题一定出在 evict 和 Add 方法的组合中。

==================
WARNING: DATA RACE
Read at 0x00c4201c81e0 by goroutine 215:
  runtime.mapaccess2_faststr()
      /usr/local/go/src/runtime/hashmap_fast.go:297 +0x0
  github.com/raintank/metrictank/mdata/cache.(*CCache).Add()
      /home/mst/go/src/github.com/raintank/metrictank/mdata/cache/cache.go:77 +0xaa

Previous write at 0x00c4201c81e0 by goroutine 155:
  runtime.mapdelete()
      /usr/local/go/src/runtime/hashmap.go:558 +0x0
  github.com/raintank/metrictank/mdata/cache.(*CCache).evict()
      /home/mst/go/src/github.com/raintank/metrictank/mdata/cache/cache.go:97 +0x30e
  github.com/raintank/metrictank/mdata/cache.(*CCache).evictLoop()
      /home/mst/go/src/github.com/raintank/metrictank/mdata/cache/cache.go:70 +0xb3

Goroutine 215 (running) created at:
  github.com/raintank/metrictank/api.(*Server).getSeries()
      /home/mst/go/src/github.com/raintank/metrictank/api/dataprocessor.go:442 +0x17c9
  github.com/raintank/metrictank/api.(*Server).getTarget()
      /home/mst/go/src/github.com/raintank/metrictank/api/dataprocessor.go:331 +0x9c3
  github.com/raintank/metrictank/api.(*Server).getTargetsLocal.func1()
      /home/mst/go/src/github.com/raintank/metrictank/api/dataprocessor.go:284 +0xa9

Goroutine 155 (running) created at:
  github.com/raintank/metrictank/mdata/cache.NewCCache()
      /home/mst/go/src/github.com/raintank/metrictank/mdata/cache/cache.go:63 +0x12f
  main.main()
      /home/mst/go/src/github.com/raintank/metrictank/metrictank.go:388 +0x246c
==================

Answer 1

我的一位同事找到了答案:

调用 NewCCache() 后，我按值(包括锁)复制了返回的变量，然后在副本上调用了 Add()，同时 evictLoop() go 例程仍然引用旧副本。所以他们在锁的不同副本上操作:)