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    SubjectRe: Recursion bug in -rt
    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;
    > >
    > >
    >
    -
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