illumos gate

The zio pipeline relies on ZIO_PIPELINE_STOP and ZIO_PIPELINE_CONTINUE to
direct the pipeline into the next stage. Unfortunately the pipeline can call
into the vdev layer which also uses these same directives without knowledge of
whether or not the zio going through the pipeline has changed.

This can lead to programming errors because at the lowest level of the issue
pipeline the zio may change because of our queuing logic:

static int
zio_vdev_io_start(zio_t *zio) {
<snip>

if ((zio = vdev_queue_io(zio)) == NULL)
                        return (ZIO_PIPELINE_STOP);

if (!vdev_accessible(vd, zio)) {
                        zio->io_error = SET_ERROR(ENXIO);
                        zio_interrupt(zio);
                        return (ZIO_PIPELINE_STOP);
                }
        }

return (vd->vdev_ops->vdev_op_io_start(zio));
}

In the code above the zio that is used to call into the vdev layer may not be
the same zio that entered this pipeline stage. If the vdev layer continues the
pipeline then it will result in an old zio moving into the next stage without
being issued.