Re: Recursion bug in -rt

[David Singleton]

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.

Thanks for testing this.

David




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;
>  

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