parent: 17669:4f468b088d66 tip, windows/amd64

net benchmarks episodically crash with:

fatal error: runtime: internal error: misuse of lockOSThread/unlockOSThread

goroutine 12708 [running]:
runtime.throw(0x968710)
        src/pkg/runtime/panic.c:501 +0x67 fp=0x33224cc8
runtime.unlockOSThread()
        src/pkg/runtime/proc.c:1891 +0x3d fp=0x33224cd0
endcgo()
        src/pkg/runtime/cgocall.c:165 +0x20 fp=0x33224cd8
runtime.cgocall(0x424060, 0x33224d0c)
        src/pkg/runtime/cgocall.c:159 +0x159 fp=0x33224d04
syscall.Syscall(0x778b3918, 0x1, 0x200, 0x0, 0x0, ...)
        src/pkg/runtime/syscall_windows.goc:55 +0x4b fp=0x33224d28
syscall.Closesocket(0x200, 0x0, 0x0)
        src/pkg/syscall/zsyscall_windows_386.go:1399 +0x78 fp=0x33224d50
net.closesocket(0x200, 0x0, 0x0)
        src/pkg/net/fd_windows.go:71 +0x3a fp=0x33224d60
net.(*netFD).destroy(0x127ba180)
        src/pkg/net/fd_windows.go:360 +0x4c fp=0x33224d74
net.(*netFD).decref(0x127ba180)
        src/pkg/net/fd_windows.go:379 +0x40 fp=0x33224d80
net.(*netFD).Close(0x127ba180, 0x0, 0x0)
        src/pkg/net/fd_windows.go:421 +0x7b fp=0x33224d8c
net.(*conn).Close(0x12674908, 0x0, 0x0)
        src/pkg/net/net.go:139 +0x92 fp=0x33224da0
net/http.(*persistConn).closeLocked(0x125e6c30)
        src/pkg/net/http/transport.go:944 +0x3f fp=0x33224db0
net/http.(*persistConn).close(0x125e6c30)
        src/pkg/net/http/transport.go:938 +0x57 fp=0x33224db8
net/http.(*Transport).putIdleConn(0x125e81e0, 0x125e6c30, 0x1fc000)
        src/pkg/net/http/transport.go:323 +0x1ac fp=0x33224e20
net/http.func┬╖020(0x0, 0x0)
        src/pkg/net/http/transport.go:757 +0x60 fp=0x33224e3c
net/http.(*bodyEOFSignal).condfn(0x126b6930, 0x1fc000, 0x125e0020)
        src/pkg/net/http/transport.go:1024 +0x87 fp=0x33224e54
net/http.(*bodyEOFSignal).Read(0x126b6930, 0x12f1520d, 0x5f3, 0x5f3, 0x0, ...)
        src/pkg/net/http/transport.go:996 +0x1ff fp=0x33224e8c
bytes.(*Buffer).ReadFrom(0x12c95540, 0x1fde68, 0x126b6930, 0xd, 0x0, ...)
        src/pkg/bytes/buffer.go:169 +0x1da fp=0x33224ef4
io/ioutil.readAll(0x1fde68, 0x126b6930, 0x200, 0x0, 0x0, ...)
        src/pkg/io/ioutil/ioutil.go:32 +0x13f fp=0x33224f40
io/ioutil.ReadAll(0x1fde68, 0x126b6930, 0x0, 0x0, 0x0, ...)
        src/pkg/io/ioutil/ioutil.go:41 +0x6a fp=0x33224f68
net/http_test.func┬╖112()
        src/pkg/net/http/serve_test.go:1919 +0x12d fp=0x33224fd0
runtime.goexit()
        src/pkg/runtime/proc.c:1364 fp=0x33224fd4
created by net/http_test.benchmarkClientServerParallel
        src/pkg/net/http/serve_test.go:1930 +0x16c