fs: Introduce FOP_ASYNC_LOCK

Some lock managers (NLM, kNFSD) fastidiously avoid blocking their
kernel threads while servicing blocking locks.  If a filesystem supports
asynchronous lock requests those lock managers can use notifications to
quickly inform clients they have acquired a file lock.

Historically, only posix_lock_file() was capable of supporting asynchronous
locks so the check for support was simply file_operations->lock(), but with
recent changes in DLM, both GFS2 and OCFS2 also support asynchronous locks
and have started signalling their support with EXPORT_OP_ASYNC_LOCK.

We recently noticed that those changes dropped the checks for whether a
filesystem simply defaults to posix_lock_file(), so async lock
notifications have not been attempted for NLM and NFSv4.1+ for most
filesystems.  While trying to fix this it has become clear that testing
both the export flag combined with testing ->lock() creates quite a
layering mess.  It seems appropriate to signal support with a fop_flag.

Add FOP_ASYNC_LOCK so that filesystems with ->lock() can signal their
capability to handle lock requests asynchronously.  Add a helper for
lock managers to properly test that support.

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Link: https://lore.kernel.org/r/3330d5a324abe2ce9c1dafe89cacdc6db41945d1.1726083391.git.bcodding@redhat.com
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

include/linux/filelock.h

@@ -180,6 +180,11 @@ static inline void locks_wake_up(struct file_lock *fl)
 	wake_up(&fl->c.flc_wait);
 }
 
+static inline bool locks_can_async_lock(const struct file_operations *fops)
+{
+	return !fops->lock || fops->fop_flags & FOP_ASYNC_LOCK;
+}
+
 /* fs/locks.c */
 void locks_free_lock_context(struct inode *inode);
 void locks_free_lock(struct file_lock *fl);

include/linux/fs.h

@@ -2074,6 +2074,8 @@ struct file_operations {
 #define FOP_DIO_PARALLEL_WRITE	((__force fop_flags_t)(1 << 3))
 /* Contains huge pages */
 #define FOP_HUGE_PAGES		((__force fop_flags_t)(1 << 4))
+/* Supports asynchronous lock callbacks */
+#define FOP_ASYNC_LOCK		((__force fop_flags_t)(1 << 5))
 
 /* Wrap a directory iterator that needs exclusive inode access */
 int wrap_directory_iterator(struct file *, struct dir_context *,