Recursion bug in -rt

[Dinakar Guniguntala]

Hi David,

I hit this bug with -rt22-rf11

==========================================
[ BUG: lock recursion deadlock detected! |
------------------------------------------
already locked:  [f7abbc94] {futex}
.. held by:          testpi-3: 4595 [f7becdd0,  59]
... acquired at:               futex_wait_robust+0x142/0x1f3
------------------------------
| showing all locks held by: |  (testpi-3/4595 [f7becdd0,  59]):
------------------------------

#001:             [f7abbc94] {futex}
... acquired at:               futex_wait_robust+0x142/0x1f3

-{current task's backtrace}----------------->
 [<c0103e04>] dump_stack+0x1e/0x20 (20)
 [<c0136bc2>] check_deadlock+0x2d7/0x334 (44)
 [<c01379bc>] task_blocks_on_lock+0x2c/0x224 (36)
 [<c03f29c5>] __down_interruptible+0x37c/0x95d (160)
 [<c013aebf>] down_futex+0xa3/0xe7 (40)
 [<c013ebc5>] futex_wait_robust+0x142/0x1f3 (72)
 [<c013f35c>] do_futex+0x9a/0x109 (40)
 [<c013f4dd>] sys_futex+0x112/0x11e (68)
 [<c0102f03>] sysenter_past_esp+0x54/0x75 (-8116)
------------------------------
| showing all locks held by: |  (testpi-3/4595 [f7becdd0,  59]):
------------------------------

#001:             [f7abbc94] {futex}
... acquired at:               futex_wait_robust+0x142/0x1f3

---------------------------------------------------------------------

futex.c -> futex_wait_robust

        if ((curval & FUTEX_PID) == current->pid) {
                ret = -EAGAIN;
                goto out_unlock;
        }

rt.c    -> down_futex

        if (!owner_task || owner_task == current) {
                up(sem);
                up_read(&current->mm->mmap_sem);
                return -EAGAIN;
        }

I noticed that both the above checks below have been removed in your
patch. I do understand that the futex_wait_robust path has been
made similar to the futex_wait path, but I think we are not taking
PI into consideration. Basically it looks like we still need to check
if the current task has become owner. or are we missing a lock somewhere ?

I added the down_futex check above and my test has been
running for hours without the oops. Without this check it
used to oops within minutes.

Patch that works for me attached below.  Thoughts?

        -Dinakar


Index: linux-2.6.14-rt22-rayrt5/kernel/rt.c
===================================================================
--- linux-2.6.14-rt22-rayrt5.orig/kernel/rt.c	2005-12-15 02:15:13.000000000 +0530
+++ linux-2.6.14-rt22-rayrt5/kernel/rt.c	2005-12-15 02:18:29.000000000 +0530
@@ -3001,7 +3001,7 @@
 	 * if the owner can't be found return try again.
 	 */
 
-	if (!owner_task) {
+	if (!owner_task || owner_task == current) {
 		up(sem);
 		up_read(&current->mm->mmap_sem);
 		return -EAGAIN;