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    SubjectRe: Recursion bug in -rt
    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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