CFR: New NFS Lock Manager

Erik Trulsson ertr1013 at
Fri Mar 21 12:54:14 PDT 2008

On Fri, Mar 21, 2008 at 12:07:25PM -0700, Julian Elischer wrote:
> 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
> Bikeshed.

No, it is much older than that, but is indeed the opposite of a bikeshed.
See the FreeBSD FAQ at
for the whole story.

> 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.

Yup. "nuclear reactor", indeed.

> 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.

<Insert your favourite quote here.>
Erik Trulsson
ertr1013 at

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