From: Josef Bacik <jbacik@redhat.com>
Date: Mon, 20 Apr 2009 15:52:31 -0400
Subject: [fs] fix softlockup in posix_locks_deadlock
Message-id: 1240257151-32296-1-git-send-email-jbacik@redhat.com
O-Subject: [PATCH] [RHEL5.4] fix softlockup in posix_locks_deadlock
Bugzilla: 476659
RH-Acked-by: Peter Staubach <staubach@redhat.com>
RH-Acked-by: Eric Sandeen <sandeen@redhat.com>

This is in reference to bz 476659, and is a backport of

commit 97855b49b6bac0bd25f16b017883634d13591d00
Author: J. Bruce Fields <bfields@citi.umich.edu>
Date:   Tue Oct 30 11:20:02 2007 -0400

    locks: fix possible infinite loop in posix deadlock detection

    It's currently possible to send posix_locks_deadlock() into an infinite
    loop (under the BKL).

    For now, fix this just by bailing out after a few iterations.  We may
    want to fix this in a way that better clarifies the semantics of
    deadlock detection.  But that will take more time, and this minimal fix
    is probably adequate for any realistic scenario, and is simple enough to
    be appropriate for applying to stable kernels now.

    Thanks to George Davis for reporting the problem.

    Cc: "George G. Davis" <gdavis@mvista.com>
    Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
    Acked-by: Alan Cox <alan@redhat.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

This has been tested by the customer.
Signed-off-by: Josef Bacik <jbacik@redhat.com>

diff --git a/fs/locks.c b/fs/locks.c
index 6aa1018..1f2d6ef 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -713,11 +713,20 @@ EXPORT_SYMBOL(posix_test_lock);
  * Note: the above assumption may not be true when handling lock requests
  * from a broken NFS client. But broken NFS clients have a lot more to
  * worry about than proper deadlock detection anyway... --okir
+ *
+ * However, the failure of this assumption (also possible in the case of
+ * multiple tasks sharing the same open file table) also means there's no
+ * guarantee that the loop below will terminate.  As a hack, we give up
+ * after a few iterations.
  */
+
+#define MAX_DEADLK_ITERATIONS 10
+
 static int posix_locks_deadlock(struct file_lock *caller_fl,
 				struct file_lock *block_fl)
 {
 	struct list_head *tmp;
+	int i = 0;
 
 next_task:
 	if (posix_same_owner(caller_fl, block_fl))
@@ -725,6 +734,8 @@ next_task:
 	list_for_each(tmp, &blocked_list) {
 		struct file_lock *fl = list_entry(tmp, struct file_lock, fl_link);
 		if (posix_same_owner(fl, block_fl)) {
+			if (i++ > MAX_DEADLK_ITERATIONS)
+				return 0;
 			fl = fl->fl_next;
 			block_fl = fl;
 			goto next_task;