Re: [PATCH v1 0/4] Fixup NLM and kNFSD file lock callbacks

[Jeff Layton]

On Thu, 2024-09-12 at 13:32 +0200, Christian Brauner wrote:
> On Thu, Sep 12, 2024 at 07:08:07AM GMT, Jeff Layton wrote:
> > On Wed, 2024-09-11 at 15:42 -0400, Benjamin Coddington wrote:
> > > Last year both GFS2 and OCFS2 had some work done to make their locking more
> > > robust when exported over NFS.  Unfortunately, part of that work caused both
> > > NLM (for NFS v3 exports) and kNFSD (for NFSv4.1+ exports) to no longer send
> > > lock notifications to clients.
> > > 
> > > This in itself is not a huge problem because most NFS clients will still
> > > poll the server in order to acquire a conflicted lock, but now that I've
> > > noticed it I can't help but try to fix it because there are big advantages
> > > for setups that might depend on timely lock notifications, and we've
> > > supported that as a feature for a long time.
> > > 
> > > Its important for NLM and kNFSD that they do not block their kernel threads
> > > inside filesystem's file_lock implementations because that can produce
> > > deadlocks.  We used to make sure of this by only trusting that
> > > posix_lock_file() can correctly handle blocking lock calls asynchronously,
> > > so the lock managers would only setup their file_lock requests for async
> > > callbacks if the filesystem did not define its own lock() file operation.
> > > 
> > > However, when GFS2 and OCFS2 grew the capability to correctly
> > > handle blocking lock requests asynchronously, they started signalling this
> > > behavior with EXPORT_OP_ASYNC_LOCK, and the check for also trusting
> > > posix_lock_file() was inadvertently dropped, so now most filesystems no
> > > longer produce lock notifications when exported over NFS.
> > > 
> > > I tried to fix this by simply including the old check for lock(), but the
> > > resulting include mess and layering violations was more than I could accept.
> > > There's a much cleaner way presented here using an fop_flag, which while
> > > potentially flag-greedy, greatly simplifies the problem and grooms the
> > > way for future uses by both filesystems and lock managers alike.
> > > 
> > > Criticism welcomed,
> > > Ben
> > > 
> > > Benjamin Coddington (4):
> > >   fs: Introduce FOP_ASYNC_LOCK
> > >   gfs2/ocfs2: set FOP_ASYNC_LOCK
> > >   NLM/NFSD: Fix lock notifications for async-capable filesystems
> > >   exportfs: Remove EXPORT_OP_ASYNC_LOCK
> > > 
> > >  Documentation/filesystems/nfs/exporting.rst |  7 -------
> > >  fs/gfs2/export.c                            |  1 -
> > >  fs/gfs2/file.c                              |  2 ++
> > >  fs/lockd/svclock.c                          |  5 ++---
> > >  fs/nfsd/nfs4state.c                         | 19 ++++---------------
> > >  fs/ocfs2/export.c                           |  1 -
> > >  fs/ocfs2/file.c                             |  2 ++
> > >  include/linux/exportfs.h                    | 13 -------------
> > >  include/linux/filelock.h                    |  5 +++++
> > >  include/linux/fs.h                          |  2 ++
> > >  10 files changed, 17 insertions(+), 40 deletions(-)
> > > 
> > 
> > Thanks for fixing this up, Ben!
> > 
> > Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
> 
> It might be a bit late for v6.12 so I would stuff this into a branch for
> v6.13. Sound ok?

Ok. I figured Chuck would take this set, but I guess it is more VFS-y.

I think this is reasonably safe though, so if Ben needs it before then,
we could pull it in sooner.
-- 
Jeff Layton <jlayton@xxxxxxxxxx>