CFR: New NFS Lock Manager
julian at elischer.org
Fri Mar 21 12:07:45 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.
> On Fri, Mar 21, 2008 at 3:27 AM, Doug Rabson <dfr at rabson.org> 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 http://people.freebsd.org/~dfr/lockd-RC1-20032008.diff
>> 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
>> * 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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