locks: provide a file lease method enabling cluster-coherent leases

Currently leases are only kept locally, so there's no way for a distributed
filesystem to enforce them against multiple clients.  We're particularly
interested in the case of nfsd exporting a cluster filesystem, in which
case nfsd needs cluster-coherent leases in order to implement delegations
correctly.

Also add some documentation.
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

fs/locks.c

@@ -1429,7 +1429,24 @@ out:
  *	@lease: file_lock to use
  *
  *	Call this to establish a lease on the file.
- *	The fl_lmops fl_break function is required by break_lease
+ *	The (*lease)->fl_lmops->fl_break operation must be set; if not,
+ *	break_lease will oops!
+ *
+ *	This will call the filesystem's setlease file method, if
+ *	defined.  Note that there is no getlease method; instead, the
+ *	filesystem setlease method should call back to setlease() to
+ *	add a lease to the inode's lease list, where fcntl_getlease() can
+ *	find it.  Since fcntl_getlease() only reports whether the current
+ *	task holds a lease, a cluster filesystem need only do this for
+ *	leases held by processes on this node.
+ *
+ *	There is also no break_lease method; filesystems that
+ *	handle their own leases shoud break leases themselves from the
+ *	filesystem's open, create, and (on truncate) setattr methods.
+ *
+ *	Warning: the only current setlease methods exist only to disable
+ *	leases in certain cases.  More vfs changes may be required to
+ *	allow a full filesystem lease implementation.
  */
 
 int vfs_setlease(struct file *filp, long arg, struct file_lock **lease)
@@ -1437,7 +1454,10 @@ int vfs_setlease(struct file *filp, long arg, struct file_lock **lease)
 	int error;
 
 	lock_kernel();
-	error = setlease(filp, arg, lease);
+	if (filp->f_op && filp->f_op->setlease)
+		error = filp->f_op->setlease(filp, arg, lease);
+	else
+		error = setlease(filp, arg, lease);
 	unlock_kernel();
 
 	return error;

include/linux/fs.h

@@ -1122,6 +1122,7 @@ struct file_operations {
 	int (*flock) (struct file *, int, struct file_lock *);
 	ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
 	ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
+	int (*setlease)(struct file *, long, struct file_lock **);
 };
 
 struct inode_operations {