I saw this error at http://build.golang.org/log/997ef39853975e571dc95f3540609bdba78af2a2
(darwin-amd64-cheney-race)

BenchmarkFinalizerRun-4 runtime.SetFinalizer: finalizer already set
fatal error: runtime.SetFinalizer

goroutine 388 [running]:
runtime.throw(0x6d976f)
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/runtime/panic.c:464 +0x69 fp=0xaf0ed0
runtime.SetFinalizer(0x245140, 0xc2080012c0, 0x279560, 0x38b980)
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/runtime/malloc.goc:943 +0x40 fp=0xaf0f50
runtime_test.func·061()
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/runtime/mfinal_test.go:153 +0xd0 fp=0xaf0fa0
runtime.goexit()
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/runtime/proc.c:1438 fp=0xaf0fa8
created by runtime_test.BenchmarkFinalizerRun
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/runtime/mfinal_test.go:158 +0x187

goroutine 16 [chan receive]:
testing.(*B).run(0xc208101590, 0x3, 0xae2d60, 0x1, 0x1, ...)
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/testing/benchmark.go:171 +0x75
testing.RunBenchmarks(0x38ae90, 0x6dba80, 0xa1, 0xa1)
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/testing/benchmark.go:303 +0x6be
testing.Main(0x38ae90, 0x6da940, 0x51, 0x51, 0x6dba80, ...)
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/testing/testing.go:419 +0x217
main.main()
    /tmp/builder/go-build014524229/runtime/_test/_testmain.go:529 +0xdd

goroutine 8552 [semacquire]:
sync.runtime_Semacquire(0xc208120000)
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/runtime/sema.goc:199 +0x30
sync.(*WaitGroup).Wait(0xc20803a040)
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/sync/waitgroup.go:127 +0x1c1
runtime_test.BenchmarkFinalizerRun(0xc208101590)
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/runtime/mfinal_test.go:160 +0x1b2
testing.(*B).runN(0xc208101590, 0x30d40)
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/testing/benchmark.go:119 +0xc1
testing.(*B).launch(0xc208101590)
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/testing/benchmark.go:207 +0x1bb
created by testing.(*B).run
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/testing/benchmark.go:170 +0x3c

goroutine 8587 [runnable]:
runtime_test.func·061()
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/runtime/mfinal_test.go:155 +0xea
created by runtime_test.BenchmarkFinalizerRun
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/runtime/mfinal_test.go:158 +0x187

goroutine 8588 [runnable]:
runtime_test.func·061()
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/runtime/mfinal_test.go:152 +0x7e
created by runtime_test.BenchmarkFinalizerRun
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/runtime/mfinal_test.go:158 +0x187

goroutine 8589 [runnable]:
runtime_test.func·061()
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/runtime/mfinal_test.go:152 +0x7e
created by runtime_test.BenchmarkFinalizerRun
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/runtime/mfinal_test.go:158 +0x187

goroutine 1509 [runnable]:
runtime.call16(0x38b980, 0xc208121700)
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/runtime/asm_amd64.s:341
runfinq()
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/runtime/mgc0.c:2574 +0x18c
runtime.goexit()
    /Users/builder/workspace/darwin-amd64-race-cheney-dc75bcbf2692/go/src/pkg/runtime/proc.c:1438
exit status 2
FAIL    runtime 45.944s