link() not increasing link count on NFS server

[Kris Kennaway]

Timo Sirainen wrote:
> On Thu, 2007-11-15 at 12:39 +0000, Robert Watson wrote:
>>> or Solaris NFS clients. Basically, Timo (cc'ed) came up with a small test 
>>> case that seems to indicate sometimes a link() call can succeed while the 
>>> link count of the file will not increase.  If this is ran on two FreeBSD 
>>> clients from the same NFS directory, you will occasionally see "link() 
>>> succeeded, but link count=1".  I've tried both a Netapp and a FreeBSD NFS 
> ..
>> My guess, and this is just a hand-wave, is that the attribute cache in the NFS 
>> client isn't being forced to refresh, and hence you're getting the old stat 
>> data back (and perhaps there's no GETATTR on the wire, which might hint at 
>> this).  If you'd like, you can post a link to the pcap capture file and one of 
>> us can take a look, but I've found NFS RPCs to be surprisingly readable in 
>> Wireshark so you might find it sheds quite a bit of light.
> 
> Actually the point was that link() returns success even though in
> reality it fails. The fstat() was just a workaround to catch this case
> and treat link count 1 as if link() had failed with EEXIST. After that I
> had no more problems with locking.
> 
> I noticed this first because my dotlocking was failing to lock files
> properly. I also added fchown() to flush attribute cache after link()
> and before fstat(), it gives the same link count=1 reply.

Is it failing (leaving the link count unchanged, when fstat()'ed on the 
server), or is it just returning a stale link count when the *client* 
runs fstat()?

There is at least one race with the NFS attribute cache that sometimes 
returns old data to clients when they stat a file that has been recently 
modified.

Kris