我正在编写一个简单的缓存机制，它具有Add Evict和Search方法。 Search目前尚未实施，因此无需担心。
有相对大量的goroutine调用Add来添加数据，并且只有一个在evict循环中运行以驱逐数据。一旦我对它施加了一些严重的流量，Go就会说在地图metricCache上有一个并发的读写访问权限，但是我看不到它是如何发生的，因为它周围有锁。我正在使用Go 1.7。
档案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：我使用-race重新编译，现在我收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，问题必须是evict和Add方法的组合。
==================
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
==================
答案 0 :(得分：2)
我的一位同事找到了答案：
在调用NewCCache()之后，我按值（包括锁定）复制了返回的变量，然后在副本上调用了Add()，同时evictLoop() go例程仍然指的是旧的副本。所以他们在不同的锁副本上操作：）