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>

1 files changed, 22 insertions, 2 deletions

diff --git a/fs/locks.c b/fs/locks.c
index a65d85c1fdc2..94f5d8065e3a 100644
--- a/fs/locks.c
+++ b/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;