The PipelineHandler lock() and unlock() functions are documented as thread-safe, but they're not. Fix them using a mutex. Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com> Reviewed-by: Jacopo Mondi <jacopo@jmondi.org> Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>