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). > 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. -- Jeff Layton <jlayton@xxxxxxxxxx>
