----- Original Message ----- > From: "Jeff Layton" <jlayton@xxxxxxxxxx> > To: "David Howells" <dhowells@xxxxxxxxxx>, "Paulo Alcantara" <pc@xxxxxx> > Cc: viro@xxxxxxxxxxxxxxxxxx, "Steve French" <smfrench@xxxxxxxxx>, "linux-nfs" <linux-nfs@xxxxxxxxxxxxxxx>, "CIFS" > <linux-cifs@xxxxxxxxxxxxxxx>, linux-afs@xxxxxxxxxxxxxxxxxxx, ceph-devel@xxxxxxxxxxxxxxx, keyrings@xxxxxxxxxxxxxxx, > "Network Development" <netdev@xxxxxxxxxxxxxxx>, "LKML" <linux-kernel@xxxxxxxxxxxxxxx>, fweimer@xxxxxxxxxx > Sent: Tuesday, 21 April, 2020 8:30:37 AM > Subject: Re: cifs - Race between IP address change and sget()? > > On Mon, 2020-04-20 at 23:14 +0100, David Howells wrote: > > Paulo Alcantara <pc@xxxxxx> wrote: > > > > > > > > What happens if the IP address the superblock is going to changes, > > > > > > then > > > > > > another mount is made back to the original IP address? Does the > > > > > > second > > > > > > mount just pick the original superblock? > > > > > > > > > > It is going to transparently reconnect to the new ip address, SMB > > > > > share, > > > > > and cifs superblock is kept unchanged. We, however, update internal > > > > > TCP_Server_Info structure to reflect new destination ip address. > > > > > > > > > > For the second mount, since the hostname (extracted out of the UNC > > > > > path > > > > > at mount time) resolves to a new ip address and that address was > > > > > saved > > > > > earlier in TCP_Server_Info structure during reconnect, we will end up > > > > > reusing same cifs superblock as per > > > > > fs/cifs/connect.c:cifs_match_super(). > > > > > > > > Would that be a bug? > > > > > > Probably. > > > > > > I'm not sure how that code is supposed to work, TBH. > > > > Hmmm... I think there may be a race here then - but I'm not sure it can be > > avoided or if it matters. > > > > Since the address is part of the primary key to sget() for cifs, changing > > the > > IP address will change the primary key. Jeff tells me that this is > > governed > > by a spinlock taken by cifs_match_super(). However, sget() may be busy > > attaching a new mount to the old superblock under the sb_lock core vfs > > lock, > > having already found a match. > > > > Not exactly. Both places that match TCP_Server_Info objects by address > hold the cifs_tcp_ses_lock. The address looks like it gets changed in > reconn_set_ipaddr, and the lock is not currently taken there, AFAICT. I > think it probably should be (at least around the cifs_convert_address > call). I think you are right. We need the spinlock around this call too. I will send a patch to the list to add this. > > > Should the change of parameters made by cifs be effected with sb_lock held > > to > > try and avoid ending up using the wrong superblock? > > > > However, because the TCP_Server_Info is apparently updated, it looks like > > my > > original concern is not actually a problem (the idea that if a mounted > > server > > changes its IP address and then a new server comes online at the old IP > > address, it might end up sharing superblocks because the IP address is part > > of > > the key). > > > > I'm not sure we should concern ourselves with much more than just not > allowing addresses to change while matching/searching. If you're > standing up new servers at old addresses while you still have clients > are migrating, then you are probably Doing it Wrong. Agree. That is a migration process issue and not something we can/should try to address in cifs.ko. > > -- > Jeff Layton <jlayton@xxxxxxxxxx> > >
