[Cluster-devel] Re: [PATCH 1/2] dlm: initialize file_lock struct in GETLK before copying conflicting lock

[David Teigland <teigland@redhat.com>]

On Wed, Jan 21, 2009 at 06:42:39PM -0500, J. Bruce Fields wrote:
> On Wed, Jan 21, 2009 at 11:34:50AM -0500, Jeff Layton wrote:
> > dlm_posix_get fills out the relevant fields in the file_lock before
> > returning when there is a lock conflict, but doesn't clean out any of
> > the other fields in the file_lock.
> > 
> > When nfsd does a NFSv4 lockt call, it sets the fl_lmops to
> > nfsd_posix_mng_ops before calling the lower fs. When the lock comes back
> > after testing a lock on GFS2, it still has that field set. This confuses
> > nfsd into thinking that the file_lock is a nfsd4 lock.
> 
> I think of the lock system as supporting two types of objects, both
> stored in "struct lock"'s:
> 
> 	- Heavyweight locks: these have callbacks set and the filesystem
> 	  or lock manager could in theory have some private data
> 	  associated with them, so it's important that the appropriate
> 	  callbacks be called when they're released or copied.  These
> 	  are what are actually passed to posix_lock_file() and kept on
> 	  the inode lock lists.
> 	- Lightweight locks: just start, end, pid, flags, and type, with
> 	  everything zeroed out and/or ignored.
> 
> I don't see any reason why the lock passed into dlm_posix_get() needs to
> be a heavyweight lock.  In any case, if it were, then dlm_posix_get()
> would need to release the passed-in-lock before initializing the new one
> that it's returning.

It seems the nfs code is mixing those two types up a bit.  Regardless, the
rationale I see in Jeff's dlm patch is to make the two different locking paths
equivalent:

Without cfs/dlm,
nfsd4_lockt -> nfsd_test_lock -> vfs_test_lock -> posix_test_lock

With cfs/dlm,
nfsd4_lockt -> nfsd_test_lock -> vfs_test_lock -> (cfs) -> dlm_posix_get

When there's a conflict, dlm_posix_get() and posix_test_lock() should do the
same/equivalent things to the fl they are given.

posix_test_lock() does __locks_copy_lock() on the fl and then sets the pid.
dlm_posix_get() isn't using __locks_copy_lock() because it doesn't have a
conflicting file_lock to copy from.  Jeff's patch does nearly the same thing
using locks_init_lock() plus the existing assignments.  But, I think the best
solution may be for dlm_posix_get() to set up a new lightweight file_lock with
the values we need, and then call __locks_copy_lock() with it, just like
posix_test_lock().

Dave