CFR: New NFS Lock Manager

Julian Elischer julian at
Fri Mar 21 12:07:26 PDT 2008

Kip Macy wrote:
> I think that for most of us this is the "nuclear reactor" in phk's
> bikeshed story :)

The Nuclear reactor is from the SVN guys talk on "poisonous 
personalities in open software projects" I think. where
they declared it to be the logically opposite corollary of the

Doug, As a long term developer with a good track record,
most of us are just going to say "probably works fine and I
have no idea about that stuff" and leave it at that.

Who do you think has any idea about this stuff?

> I had a tested and re-factored tcp_output which got essentially no comments.
>        -Kip
> On Fri, Mar 21, 2008 at 3:27 AM, Doug Rabson <dfr at> wrote:
>> As I mentioned previously, I have been working on a brand new NFS Lock
>>  Manager which runs in kernel mode and uses the normal local locking
>>  infrastructure for its state. I'm currently trying to tie up the last
>>  few loose ends before committing this work to current. You can find a
>>  snapshot of this code at
>>  .
>>  To try it out, take a recent current (I last merged with current on
>>  20th March) and apply the patch. Build a kernel with the NFSLOCKD
>>  option and add '-k' to 'rpc_lockd_flags' in rc.conf. You will need to
>>  build and install at least a new libc and rpc.lockd.
>>  At this point, it would be useful to get some extra eyes to look over
>>  my changes. In particular the following:
>>  1. Choice of syscall number - I found one spare next to the NFS
>>  syscall and took that. The new syscall is listed in the FBSD_1.1
>>  namespace, possibly it should be somewhere else.
>>  2. ABI compatibility - I extended the flock structure by one member
>>  (adding l_sysid). I have added new operations to fcntl to support the
>>  new extended structure, leaving the old operations in place to work on
>>  the old structure. The kernel translates old to new and vice versa. No
>>  attempt is made to allow a new userland to work with an old kernel.
>>  3. The local lock manager has had a complete rewrite to support
>>  required features. The new local lock manager supports a more flexible
>>  model of lock ownership (which can support remote lock owners). I have
>>  replaced the inadequate deadlock detection code with a new (and fast)
>>  graph based system. Using the deadlock graph, I was able to avoid the
>>  'thundering herd' issues the old lock code had when many processes
>>  were contending for the same locked region. Given the extent of the
>>  changes, wider testing and review would be extremely welcome.
>>  4. The NFS lock manager itself is brand new code and as such ought to
>>  be reviewed. I have also ported the userland sunrpc code to run in the
>>  kernel environment which may prove useful in future.
>>  Highlights include:
>>  * Thread-safe kernel RPC client - many threads can use the same RPC
>>  client handle safely with replies being de-multiplexed at the socket
>>  upcall (typically driven directly by the NIC interrupt) and handed off
>>  to whichever thread matches the reply. For UDP sockets, many RPC
>>  clients can share the same socket. This allows the use of a single
>>  privileged UDP port number to talk to an arbitrary number of remote
>>  hosts.
>>  * Single-threaded kernel RPC server. Adding support for multi-threaded
>>  server would be relatively straightforward and would follow
>>  approximately the Solaris KPI. A single thread should be sufficient
>>  for the NLM since it should rarely block in normal operation.
>>  * Kernel mode NLM server supporting cancel requests and granted
>>  callbacks. I've tested the NLM server reasonably extensively - it
>>  passes both my own tests and the NFS Connectathon locking tests
>>  running on Solaris, Mac OS X and Ubuntu Linux.
>>  * Userland NLM client supported. While the NLM server doesn't have
>>  support for the local NFS client's locking needs, it does have to
>>  field async replies and granted callbacks from remote NLMs that the
>>  local client has contacted. We relay these replies to the userland
>>  rpc.lockd over a local domain RPC socket.
>>  * IPv6 should be supported but has not been tested since I've been
>>  unable to get IPv6 to work properly with the Parallels virtual
>>  machines that I've been using for development.
>>  * Robust deadlock detection for the local lock manager. In particular
>>  it will detect deadlocks caused by a lock request that covers more
>>  than one blocking request. As required by the NLM protocol, all
>>  deadlock detection happens synchronously - a user is guaranteed that
>>  if a lock request isn't rejected immediately, the lock will eventually
>>  be granted. The old system allowed for a 'deferred deadlock' condition
>>  where a blocked lock request could wake up and find that some other
>>  deadlock-causing lock owner had beaten them to the lock.
>>  * Since both local and remote locks are managed by the same kernel
>>  locking code, local and remote processes can safely use file locks for
>>  mutual exclusion. Local processes have no fairness advantage compared
>>  to remote processes when contending to lock a region that has just
>>  been unlocked - the local lock manager enforces a strict first-come
>>  first-served model for both local and remote lockers.
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