Re: [RFC][PATCH] nfs: support legacy NFS flock behavior via mount option

[Trond Myklebust <Trond.Myklebust@netapp.com>]

On Wed, 2010-09-08 at 08:23 +1000, Neil Brown wrote:
> On Tue, 07 Sep 2010 10:17:19 -0400
> Trond Myklebust <Trond.Myklebust@netapp.com> wrote:
> 
> > On Mon, 2010-09-06 at 18:03 +0530, Suresh Jayaraman wrote:
> > > NFS clients since 2.6.12 support flock()locks by emulating the
> > > BSD-style locks in terms of POSIX byte range locks. So the NFS client
> > > does not allow to lock the same file using both flock() and fcntl
> > > byte-range locks.
> > > 
> > > For some Windows applications which seem to use both share mode locks
> > > (flock()) and fcntl byte range locks sequentially on the same file,
> > > the locking is failing as the lock has already been acquired. i.e. the
> > > flock mapped as posix locks collide with actual byte range locks from
> > > the same process. The problem was observed on a setup with Windows
> > > clients accessing Excel files on a Samba exported share which is
> > > originally a NFS mount from a NetApp filer. Since kernels < 2.6.12 does
> > > not support flock, what was working (as flock locks were local) in
> > > older kernels is not working with newer kernels.
> > > 
> > > This could be seen as a bug in the implementation of the windows
> > > application or a NFS client regression, but that is debatable.
> > > In the spirit of not breaking existing setups, this patch adds mount
> > > options "flock=local" that enables older flock behavior and
> > > "flock=fcntl" that allows the current flock behavior.
> > 
> > So instead of having a special option for flock only, what say we rather
> > introduce an option of the form
> > 
> >   -olocal_lock=
> > 
> > which can take the values 'none', 'flock', 'fcntl' (or 'posix'?) and
> > 'all'?
> 
> I observe that the NLM protocol has support for 'share' reservations.
> Requesting 'access == READ, mode==DENY_WRITE' is like a shared lock,
> and 'access = WRITE, mode== DENY_READ_WRITE' is like an exclusive lock.
> 
> As samba maps theh share reservations into flock locks, it could make sense
> for NFS to (optionally) map flock locks into share reservations.
> 
> The current Linux nfsd handles contention between these reservations entirely
> internally, but it could conceivably grow an option to map them into flock
> lock, just like samba does.
> 
> If this were at all a possible future direction, I would like to ensure that
> option names chosen now allowed for that extension.
>  flock=local and flock=fcntl naturally extends to flock=share
> 
> local_lock= doesn't really extend ... unless shared_lock=flock, but that
> seems a bit backwards.
> 
> Is that a direction we could ever want to go?

I'd be against it for several reasons:

      * Ordinary flock locks are advisory, whereas deny share
        reservations are special mandatory locks. In fact if you look at
        the Samba implementation, it uses a special 'LOCK_MAND' flag in
        addition to the usual flock() flag.
      * DENY_WRITE and DENY_BOTH share reservation modes break unlink()
        behaviour on posix systems.
      * Once the file has been opened with a given access mode, my
        interpretation of the protocol is that you cannot change the
        deny mode without closing any conflicting shares first. (Section
        8.9 says: This checking of share reservations on OPEN is done
        with no exception for an existing OPEN for the same open_owner.)

Cheers
  Trond