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 <[email protected]>
Acked-by: J. Bruce Fields <[email protected]>
Signed-off-by: Al Viro <[email protected]>

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:

fs/lockd/svclock.c

@@ -744,8 +744,20 @@ static int nlmsvc_same_owner(struct file_lock *fl1, struct file_lock *fl2)
 	return fl1->fl_owner == fl2->fl_owner && fl1->fl_pid == fl2->fl_pid;
 }
 
+/*
+ * Since NLM uses two "keys" for tracking locks, we need to hash them down
+ * to one for the blocked_hash. Here, we're just xor'ing the host address
+ * with the pid in order to create a key value for picking a hash bucket.
+ */
+static unsigned long
+nlmsvc_owner_key(struct file_lock *fl)
+{
+	return (unsigned long)fl->fl_owner ^ (unsigned long)fl->fl_pid;
+}
+
 const struct lock_manager_operations nlmsvc_lock_operations = {
 	.lm_compare_owner = nlmsvc_same_owner,
+	.lm_owner_key = nlmsvc_owner_key,
 	.lm_notify = nlmsvc_notify_blocked,
 	.lm_grant = nlmsvc_grant_deferred,
 };

fs/locks.c

@@ -521,10 +521,18 @@ locks_delete_global_locks(struct file_lock *fl)
 	spin_unlock(&file_lock_lock);
 }
 
+static unsigned long
+posix_owner_key(struct file_lock *fl)
+{
+	if (fl->fl_lmops && fl->fl_lmops->lm_owner_key)
+		return fl->fl_lmops->lm_owner_key(fl);
+	return (unsigned long)fl->fl_owner;
+}
+
 static inline void
 locks_insert_global_blocked(struct file_lock *waiter)
 {
-	hash_add(blocked_hash, &waiter->fl_link, (unsigned long)waiter->fl_owner);
+	hash_add(blocked_hash, &waiter->fl_link, posix_owner_key(waiter));
 }
 
 static inline void
@@ -757,7 +765,7 @@ static struct file_lock *what_owner_is_waiting_for(struct file_lock *block_fl)
 {
 	struct file_lock *fl;
 
-	hash_for_each_possible(blocked_hash, fl, fl_link, (unsigned long)block_fl->fl_owner) {
+	hash_for_each_possible(blocked_hash, fl, fl_link, posix_owner_key(block_fl)) {
 		if (posix_same_owner(fl, block_fl))
 			return fl->fl_next;
 	}

include/linux/fs.h

@@ -908,6 +908,7 @@ struct file_lock_operations {
 
 struct lock_manager_operations {
 	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 *);