Fixup NLM and kNFSD file lock callbacks

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(-)

2.44.0