linux-kernel - [PATCH v1 0/4] Fixup NLM and kNFSD file lock callbacks

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Message-ID: <cover.1726083391.git.bcodding@redhat.com>
Date: Wed, 11 Sep 2024 15:42:56 -0400
From: Benjamin Coddington <bcodding@...hat.com>
To: Chuck Lever <chuck.lever@...cle.com>,
	Jeff Layton <jlayton@...nel.org>,
	Amir Goldstein <amir73il@...il.com>,
	Neil Brown <neilb@...e.de>,
	Trond Myklebust <trondmy@...nel.org>,
	Anna Schumaker <anna@...nel.org>,
	Jonathan Corbet <corbet@....net>,
	Andreas Gruenbacher <agruenba@...hat.com>,
	Mark Fasheh <mark@...heh.com>,
	Joel Becker <jlbec@...lplan.org>,
	Joseph Qi <joseph.qi@...ux.alibaba.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>,
	Jan Kara <jack@...e.cz>,
	Alexander Ahring Oder Aring <aahringo@...hat.com>
Cc: linux-fsdevel@...r.kernel.org,
	linux-nfs@...r.kernel.org,
	linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	gfs2@...ts.linux.dev,
	ocfs2-devel@...ts.linux.dev
Subject: [PATCH v1 0/4] 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