From: Mikulas Patocka This patch prevents the multipath target from changing the device handler once it has been set on a device. This avoids a kernel crash that can happen when changing it. When we reload a multipath device, there are two instances of the multipath target - the first instance that is active and the second instance that is being constructed with "ctr" method. If the multipath constructor finds out that the device is using a different device handler, it detaches the existing handler and attaches a new handler. However, the first instance of the multipath target still exists and processes requests. If the first instance sends some path-management request with scsi_dh_activate and the second instance detaches the device handler while the path-management request is in flight, a crash happens. The reason for the crash is that the endio routine for the path-management request is working with structures that were freed when the handler was detached. There is no practical need to change device handlers on an active device, so this patch disables it. References: http://bugzilla.redhat.com/912245 http://bugzilla.redhat.com/902595 Signed-off-by: Mikulas Patocka Cc: stable@vger.kernel.org Acked-by: Mike Snitzer Signed-off-by: Alasdair G Kergon // FIXME Make relevant parts of bugzillas (with kernel error messages) public --- drivers/md/dm-mpath.c | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-) Index: linux/drivers/md/dm-mpath.c =================================================================== --- linux.orig/drivers/md/dm-mpath.c +++ linux/drivers/md/dm-mpath.c @@ -618,12 +618,13 @@ static struct pgpath *parse_path(struct */ r = scsi_dh_attach(q, m->hw_handler_name); if (r == -EBUSY) { - /* - * Already attached to different hw_handler: - * try to reattach with correct one. - */ - scsi_dh_detach(q); - r = scsi_dh_attach(q, m->hw_handler_name); + ti->error = "a different device handler is already attached"; + attached_handler_name = scsi_dh_attached_handler_name(q, GFP_KERNEL); + DMERR("%s: Device handler %s already attached. " + "Deactivate device to change device handler.", + p->path.dev->name, attached_handler_name); + kfree(attached_handler_name); + goto bad; } if (r < 0) {