Hi Christoph, On 9/9/2021 2:40 PM, Christoph Hellwig wrote: > On Tue, Sep 07, 2021 at 11:04:16AM +0800, Hou Tao wrote: >> Let me explain first. The reason it works is due to genl_lock_all() in netlink code. > Btw, please properly format your mail. These overly long lines are really > hard to read. Thanks for reminding. >> If the module removal happens before calling try_module_get(), nbd_cleanup() will >> >> call genl_unregister_family() first, and then genl_lock_all(). genl_lock_all() will >> >> prevent ops in nbd_connect_genl_ops() from being called, because the calling >> >> of nbd ops happens in genl_rcv() which needs to acquire cb_lock first. > Good. > >> I have checked multiple genl_ops, it seems that the reason why these genl_ops >> >> don't need try_module_get() is that these ops don't create new object through >> >> genl_ops and just control it. However genl_family_rcv_msg_dumpit() will try to >> >> call try_module_get(), but according to the history (6dc878a8ca39 "netlink: add reference of module in netlink_dump_start"), >> >> it is because inet_diag_handler_cmd() will call __netlink_dump_start(). > And now taking a step back: Why do we even need this extra module > reference? For the case where nbd_alloc_config is called from nbd_open > we obviously don't need it. In the case where it is called from > nbd_genl_connect that prevents unloading nbd when there is a configured > but not actually nbd device. Which isn't reallyed need and counter to > how other drivers work. Yes, the purpose of module ref-counting in nbd_alloc_config() is to force the user to disconnect the nbd device manually before module removal. And loop device works in the same way. If a file is attached to a loop device, an extra module reference will be taken in loop_configure() and the removal of loop module will fail. The only difference is that loop driver takes the extra ref-count by ioctl, and nbd does it through netlink. > > Did you try just removing the extra module refcounting? Yes, removing the module refcounting in nbd_alloc_config() and cleaning the nbd_config in nbd_cleanup() also work, but not sure whether or not it will break some nbd user-case which depends on the extra module reference count. I prefer to keep the extra module refcounting considering that loop driver does it as well, so what is your suggestion ? Regards, Tao > .
