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

[Jeff Layton]

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>