Hi Bruce, I'm not sure. My understanding has been that, NFSv4 does not mandate a mechanism to update clients of changes outside of any locked range. In AFS (and I think DCE DFS?) this role is played by DataVersion. If I recall correctly, David Noveck provided an errata that addresses this, that servers could use in a similar manner to DV, but I don't recall the details. Matt On Thu, Oct 1, 2020 at 5:48 PM bfields@xxxxxxxxxxxx <bfields@xxxxxxxxxxxx> wrote: > > On Mon, Jun 22, 2020 at 09:52:22AM -0400, bfields@xxxxxxxxxxxx wrote: > > On Thu, Jun 18, 2020 at 04:09:05PM -0400, bfields@xxxxxxxxxxxx wrote: > > > I probably don't understand the algorithm (in particular, how it > > > revalidates caches after a write). > > > > > > How does it avoid a race like this?: > > > > > > Start with a file whose data is all 0's and change attribute x: > > > > > > client 0 client 1 > > > -------- -------- > > > take write lock on byte 0 > > > take write lock on byte 1 > > > write 1 to offset 0 > > > change attribute now x+1 > > > write 1 to offset 1 > > > change attribute now x+2 > > > getattr returns x+2 > > > getattr returns x+2 > > > unlock > > > unlock > > > > > > take readlock on byte 1 > > > > > > At this point a getattr will return change attribute x+2, the same as > > > was returned after client 0's write. Does that mean client 0 assumes > > > the file data is unchanged since its last write? > > > > Basically: write-locking less than the whole range doesn't prevent > > concurrent writes outside that range. And the change attribute gives us > > no way to identify whether concurrent writes have happened. (At least, > > not without NFS4_CHANGE_TYPE_IS_VERSION_COUNTER.) > > > > So as far as I can tell, a client implementation has no reliable way to > > revalidate its cache outside the write-locked area--instead it needs to > > just throw out that part of the cache. > > Does my description of that race make sense? > > --b. > -- Matt Benjamin Red Hat, Inc. 315 West Huron Street, Suite 140A Ann Arbor, Michigan 48103 http://www.redhat.com/en/technologies/storage tel. 734-821-5101 fax. 734-769-8938 cel. 734-216-5309
