Re: Recursion bug in -rt

[Dinakar Guniguntala <dino@in.ibm.com>]

On Thu, Dec 15, 2005 at 11:00:49AM -0800, David Singleton wrote:
> Dinakar,
>     after further testing and investigation I believe you original 
> assessment was correct.
> The problem you are seeing is not a library problem.
> The changes to down_futex need to be reverted.  There is a new patch at
> 
> http://source.mvista.com/~dsingleton/patch-2.6.15-rc5-rt2-rf2.
> 
> that reverts the changes to down_futex.
> 

David, See my previous mail. IMO this patch is not right and
besides it does not fix the hang that I am seeing either.

I am continuing to debug the hang. Will keep you posted

	-Dinakar


> Dinakar Guniguntala wrote:
> 
> >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;
> > 
> >
>