It returns EINTR, so we need to handle it.

$ bash test/zdtm.sh --restore-sibling ns/static/env00
...
futex(0x7fc20ec92010, FUTEX_WAIT, 1, {120, 0}) = ? ERESTART_RESTARTBLOCK (Interrupted by signal)
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>