locks: add a new "lm_owner_key" lock operation

Currently, the hashing that the locking code uses to add these values
to the blocked_hash is simply calculated using fl_owner field. That's
valid in most cases except for server-side lockd, which validates the
owner of a lock based on fl_owner and fl_pid.

In the case where you have a small number of NFS clients doing a lot
of locking between different processes, you could end up with all
the blocked requests sitting in a very small number of hash buckets.

Add a new lm_owner_key operation to the lock_manager_operations that
will generate an unsigned long to use as the key in the hashtable.
That function is only implemented for server-side lockd, and simply
XORs the fl_owner and fl_pid.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 11 insertions, 5 deletions

diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index c2963a74fbc3..2db7c9e492e9 100644
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -349,6 +349,7 @@ fl_release_private:	maybe		no
 ----------------------- lock_manager_operations ---------------------------
 prototypes:
         int (*lm_compare_owner)(struct file_lock *, struct file_lock *);
+        unsigned long (*lm_owner_key)(struct file_lock *);
         void (*lm_notify)(struct file_lock *);  /* unblock callback */
         int (*lm_grant)(struct file_lock *, struct file_lock *, int);
         void (*lm_break)(struct file_lock *); /* break_lease callback */
@@ -358,16 +359,21 @@ locking rules:
 
                         inode->i_lock   file_lock_lock  may block
 lm_compare_owner:       yes[1]          maybe           no
+lm_owner_key            yes[1]          yes             no
 lm_notify:              yes             yes             no
 lm_grant:               no              no              no
 lm_break:               yes             no              no
 lm_change               yes             no              no
 
-[1]:    ->lm_compare_owner is generally called with *an* inode->i_lock held. It
-may not be the i_lock of the inode for either file_lock being compared! This is
-the case with deadlock detection, since the code has to chase down the owners
-of locks that may be entirely unrelated to the one on which the lock is being
-acquired. When doing a search for deadlocks, the file_lock_lock is also held.
+[1]:    ->lm_compare_owner and ->lm_owner_key are generally called with
+*an* inode->i_lock held. It may not be the i_lock of the inode
+associated with either file_lock argument! This is the case with deadlock
+detection, since the code has to chase down the owners of locks that may
+be entirely unrelated to the one on which the lock is being acquired.
+For deadlock detection however, the file_lock_lock is also held. The
+fact that these locks are held ensures that the file_locks do not
+disappear out from under you while doing the comparison or generating an
+owner key.
 
 --------------------------- buffer_head -----------------------------------
 prototypes: