go race detector reporting race conditions

[jchiu0]

We're from dgraph (github.com/dgraph-io/dgraph) and we have been using your lock-free hash.

One thing that puzzled us recently is why the Go race detector detects race conditions in some of our Go packages, for example "query".

go test -race github.com/dgraph-io/dgraph/query/...

We thought the lock-free hash use atomics and should not have race conditions. Are these expected? Here is the output for the above run. Any suggestions would be most welcome!

==================
WARNING: DATA RACE
Read at 0x00c42032cd90 by goroutine 69:
  github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic.(*Hash).getBucketByIndex()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic/hash.go:407 +0x131
  github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic.(*Hash).getBucketByIndex()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic/hash.go:417 +0x285
  github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic.(*Hash).getBucketByIndex()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic/hash.go:417 +0x285
  github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic.(*Hash).getBucketByIndex()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic/hash.go:417 +0x285
  github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic.(*Hash).getBucketByIndex()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic/hash.go:417 +0x285
  github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic.(*Hash).getBucketByHashCode()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic/hash.go:393 +0x66
  github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic.(*Hash).GetHC()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic/hash.go:247 +0x98
  github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic.(*Hash).Get()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic/hash.go:260 +0x6c
  github.com/dgraph-io/dgraph/posting.GetOrCreate()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/posting/lists.go:251 +0x107
  github.com/dgraph-io/dgraph/worker.processTask()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/worker/task.go:106 +0x561
  github.com/dgraph-io/dgraph/worker.ProcessTaskOverNetwork()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/worker/task.go:59 +0x5f0
  github.com/dgraph-io/dgraph/query.ProcessGraph()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/query/query.go:645 +0x12d5

Previous write at 0x00c42032cd90 by goroutine 70:
  sync/atomic.CompareAndSwapInt64()
      /usr/lib/go-1.7/src/runtime/race_amd64.s:298 +0xb
  sync/atomic.CompareAndSwapPointer()
      /usr/lib/go-1.7/src/runtime/atomic_pointer.go:67 +0x43
  github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic.(*Hash).getBucketByHashCode()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic/hash.go:393 +0x66
  github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic.(*Hash).GetHC()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic/hash.go:247 +0x98
  github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic.(*Hash).Get()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/vendor/github.com/zond/gotomic/hash.go:260 +0x6c
  github.com/dgraph-io/dgraph/posting.GetOrCreate()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/posting/lists.go:251 +0x107
  github.com/dgraph-io/dgraph/worker.processTask()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/worker/task.go:106 +0x561
  github.com/dgraph-io/dgraph/worker.ProcessTaskOverNetwork()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/worker/task.go:59 +0x5f0
  github.com/dgraph-io/dgraph/query.ProcessGraph()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/query/query.go:645 +0x12d5

Goroutine 69 (running) created at:
  github.com/dgraph-io/dgraph/query.ProcessGraph()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/query/query.go:692 +0x4db

Goroutine 70 (finished) created at:
  github.com/dgraph-io/dgraph/query.ProcessGraph()
      /home/jchiu/go/src/github.com/dgraph-io/dgraph/query/query.go:692 +0x4db
==================

[zond]

Good catch!

I wrote this lib before I knew about the race detector (maybe before it existed?).

I ran the unit tests with -race, and found a lot of errors, and I'm moving through them quite quickly now since I don't have to change any logic, just add more atomic.Store/LoadXXX calls.

A new version will come very soon.

[zond]

Fixed with 0c6a291

[jchiu0]

Thanks for the very quick fix!! Let us try it out.