locks: skip deadlock detection on FL_FILE_PVT locks

It's not really feasible to do deadlock detection with FL_FILE_PVT
locks since they aren't owned by a single task, per-se. Deadlock
detection also tends to be rather expensive so just skip it for
these sorts of locks.

Also, add a FIXME comment about adding more limited deadlock detection
that just applies to ro -> rw upgrades, per Andy's request.

Cc: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Jeff Layton <jlayton@redhat.com>

fs/locks.c

@@ -564,7 +564,7 @@ static void __locks_insert_block(struct file_lock *blocker,
 	BUG_ON(!list_empty(&waiter->fl_block));
 	waiter->fl_next = blocker;
 	list_add_tail(&waiter->fl_block, &blocker->fl_block);
-	if (IS_POSIX(blocker))
+	if (IS_POSIX(blocker) && !IS_FILE_PVT(blocker))
 		locks_insert_global_blocked(waiter);
 }
 
@@ -757,8 +757,16 @@ EXPORT_SYMBOL(posix_test_lock);
  * Note: the above assumption may not be true when handling lock
  * requests from a broken NFS client. It may also fail in the presence
  * of tasks (such as posix threads) sharing the same open file table.
- *
  * To handle those cases, we just bail out after a few iterations.
+ *
+ * For FL_FILE_PVT locks, the owner is the filp, not the files_struct.
+ * Because the owner is not even nominally tied to a thread of
+ * execution, the deadlock detection below can't reasonably work well. Just
+ * skip it for those.
+ *
+ * In principle, we could do a more limited deadlock detection on FL_FILE_PVT
+ * locks that just checks for the case where two tasks are attempting to
+ * upgrade from read to write locks on the same inode.
  */
 
 #define MAX_DEADLK_ITERATIONS 10
@@ -781,6 +789,13 @@ static int posix_locks_deadlock(struct file_lock *caller_fl,
 {
 	int i = 0;
 
+	/*
+	 * This deadlock detector can't reasonably detect deadlocks with
+	 * FL_FILE_PVT locks, since they aren't owned by a process, per-se.
+	 */
+	if (IS_FILE_PVT(caller_fl))
+		return 0;
+
 	while ((block_fl = what_owner_is_waiting_for(block_fl))) {
 		if (i++ > MAX_DEADLK_ITERATIONS)
 			return 0;