Hi Christoph, Any comments for this patch ? On 9/7/2021 11:04 AM, Hou Tao wrote: > Hi, > > On 9/6/2021 6:25 PM, Christoph Hellwig wrote: >> On Mon, Sep 06, 2021 at 06:08:54PM +0800, Hou Tao wrote: >>>>> + if (!try_module_get(THIS_MODULE)) >>>>> + return ERR_PTR(-ENODEV); >>>> try_module_get(THIS_MODULE) is an indicator for an unsafe pattern. If >>>> we don't already have a reference it could never close the race. >>>> >>>> Looking at the callers: >>>> >>>> - nbd_open like all block device operations must have a reference >>>> already. >>> Yes. nbd_open() has already taken a reference in dentry_open(). >>>> - for nbd_genl_connect I'm not an expert, but given that struct >>>> nbd_genl_family has a module member I suspect the networkinh >>>> code already takes a reference. >>> That was my original though, but the fact is netlink code doesn't take a module reference >>> >>> in genl_family_rcv_msg_doit() and netlink uses genl_lock_all() to serialize between module removal >>> >>> and nbd_connect_genl_ops calling, so I think use try_module_get() is OK here. >> How it this going to work? If there was a race you just shortened it, >> but it can still happen before you call try_module_get. So I think we >> need to look into how the netlink calling conventions are supposed to >> look and understand the issues there first. >> . > Let me explain first. The reason it works is due to genl_lock_all() in netlink code. > > 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. > > > process A process B > > module removal > > genl_unregister_family() > > genl_lock_all() > > down_write(&cb_lock) > > receive a new netlink message > > genl_rcv() > > // will wait for the removal of nbd ops > > down_read(&cb_lock) > > If nbd_alloc_config() happens before the module removal, genl_rcv() must > > have been acquired cb_lock & genl_mutex, so nbd_cleanup() will block in > > genl_unregister_family(). When nbd_alloc_config() calls try_module_get(), > > it will find out the module is dying, so fail nbd_genl_connect(). > > > process A process B > > a new netlink message > > genl_rcv() > > down_read(&cb_lock) > > mutex_lock(&genl_mutex) > > nbd_genl_connect() > > nbd_alloc_config() > > module removal > > genl_unregister_family > > // module is dying, so fail > > try_module_get() > > genl_lock_all() > > // wait for the completion of nbd ops > > down_write(&cb_lock) > > 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(). > > Regards, > > Tao > > > .
