link() not increasing link count on NFS server
gavin.atkinson at ury.york.ac.uk
Fri Nov 16 06:47:20 PST 2007
On Thu, 2007-11-15 at 14:05 +0000, Robert Watson wrote:
> On Thu, 15 Nov 2007, 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.
> Indeed, and inspection of nfs_vnops.c:nfs_link(): finds:
> 1772 /*
> 1773 * Kludge: Map EEXIST => 0 assuming that it is a reply to a retry.
> 1774 */
> 1775 if (error == EEXIST)
> 1776 error = 0;
> 1777 return (error);
> Neither Linux nor Solaris appears to have this logic in the client. I assume
> this is, as suggested, to work around UDP retransmissions where the reply is
> lost rather than the request. It appears to exist in revision 1.1 of
> nfs_vnops.c, so came in with 4.4BSD in the initial import, but doesn't appear
> in NetBSD so I'm guessing they've removed it. It could well be we should be
> doing the same.
Small point - NetBSD does seem to still have this code, but they have
factored it out into a nfs_linkrpc function. I'm not saying it's
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