Detected by experimental deadlock detector in cl/77450043.

==================
WARNING: DEADLOCK
Goroutine 999 lock mutex 11 while holding mutex 11:
  sync.(*RWMutex).RLock()
      src/pkg/sync/rwmutex.go:42 +0xa2
  expvar.(*Map).String()
      src/pkg/expvar/expvar.go:106 +0x77
  expvar.TestMapCounter()
      src/pkg/expvar/expvar_test.go:113 +0x53f
  testing.tRunner()
      src/pkg/testing/testing.go:422 +0x121

Mutex 11 was previously locked here:
  sync.(*RWMutex).RLock()
      src/pkg/sync/rwmutex.go:42 +0xa2
  expvar.(*Map).Do()
      src/pkg/expvar/expvar.go:203 +0x54
  expvar.(*Map).String()
      src/pkg/expvar/expvar.go:117 +0x1b4
  expvar.TestMapCounter()
      src/pkg/expvar/expvar_test.go:113 +0x53f
  testing.tRunner()
      src/pkg/testing/testing.go:422 +0x121
==================

In fact the following program crashes with "fatal error: all goroutines are asleep
- deadlock!":
http://play.golang.org/p/1uvDCFc0Vx

The problem is in the Map.String function that locks v.mu recursively (the second time
in v.Do):

func (v *Map) String() string {
    v.mu.RLock()
    defer v.mu.RUnlock()
    var b bytes.Buffer
    fmt.Fprintf(&b, "{")
    first := true
    v.Do(func(kv KeyValue) {
        if !first {
            fmt.Fprintf(&b, ", ")
        }
        fmt.Fprintf(&b, "\"%s\": %v", kv.Key, kv.Value)
        first = false
    })
    fmt.Fprintf(&b, "}")
    return b.String()
}

sync.RWMutex is not recursive for readers (and for writers)
.