Crash on darwin-amd64-race-cheney builder:
http://build.golang.org/log/e898a13ebbdd21ef4bba14e8e1c6d4be32b7bbe1

SIGSEGV: segmentation violation
PC=0x34b441

invalid spdelta 1404280 45
runtime: unexpected return pc for __tsan_acquire called from 0xc210446c60000000
__tsan_acquire()
    ?:0 +0x7

goroutine 606 [running]:
runtime.asmcgocall(0x3403ba, 0xdb2eb8)
    /usr/local/go/src/pkg/runtime/asm_amd64.s:619 +0x2d fp=0xdb2e58
runtime.cgocall(0x3403ba, 0xdb2eb8)
    /usr/local/go/src/pkg/runtime/cgocall.c:103 +0x45 fp=0xdb2ea0
runtime/race._Cfunc___tsan_acquire(0x1991008, 0x8f6428)
    runtime/race/_obj/_cgo_defun.c:43 +0x31 fp=0xdb2eb8
runtime/race.Acquire(0x1991008, 0x8f6428)
    /Users/builder/workspace/darwin-amd64-race-cheney-60eb79a2cd0f/go/src/pkg/runtime/race/race.go:93 +0x31 fp=0xdb2ed0
runtime.raceacquireg(0xc210446c60, 0x8f6428)
    /usr/local/go/src/pkg/runtime/race.c:261 +0x62 fp=0xdb2ee8
runtime.raceacquire(0x8f6428)
    /usr/local/go/src/pkg/runtime/race.c:252 +0x35 fp=0xdb2f00
endcgo()
    /usr/local/go/src/pkg/runtime/cgocall.c:171 +0x63 fp=0xdb2f10
runtime.cgocall(0x34043a, 0xdb2f70)
    /usr/local/go/src/pkg/runtime/cgocall.c:155 +0x181 fp=0xdb2f58
runtime/race._Cfunc___tsan_go_end(0x1991008)
    runtime/race/_obj/_cgo_defun.c:97 +0x31 fp=0xdb2f70
runtime/race.GoEnd(0x1991008)
    /Users/builder/workspace/darwin-amd64-race-cheney-60eb79a2cd0f/go/src/pkg/runtime/race/race.go:89 +0x27 fp=0xdb2f80
runtime.racegoend()
    /usr/local/go/src/pkg/runtime/race.c:175 +0x42 fp=0xdb2f90
runtime.goexit()
    /usr/local/go/src/pkg/runtime/proc.c:1368 +0x9 fp=0xdb2fa0
created by os/exec.(*Cmd).Start
    /usr/local/go/src/pkg/os/exec/exec.go:277 +0xd99