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    Subject[BUG 2.6.25-rc3] scheduler/hotplug: some processes are dealocked when cpu is set to offline
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    When i run cpu hotplug stress test on a system with 16 logical cpus,
    some processes are deadlocked, it can be regenerated on 2.6.25-rc2
    and 2.6.25-rc3.

    The kernel thread [watchdog/1] isn't reaped by its parent when cpu #1
    is set to offline, so that [kstopmachine] sleeps for ever.

    After i investigated, debugged and analyzed it, i think it is a scheduler
    specific issue, i noticed someone submitted a patch which mentioned
    kthread_should_stop calling rule:

    "In order to safely use kthread_stop() for kthread, there is a requirement
    on how its main loop has to be orginized. Namely, the sequence of
    events that lead to kthread being blocked (schedule()) has to be
    ordered as follows:

    set_current_state(TASK_INTERRUPTIBLE);
    if (kthread_should_stop()) break;
    schedule() or similar.

    set_current_state() implies a full memory barrier. kthread_stop()
    has a matching barrier right after an update of kthread_stop_info.k
    and before kthread's wakeup."

    I don't think it is a good programming paradigm because it will result
    in some issues very easily, why can't we provide a simple wrapper for it?

    This issue seems such one, but i tried to change it to follow this rule but
    the issue is still there.

    Why isn't the kernel thread [watchdog/1] reaped by its parent? its state
    is TASK_RUNNING with high priority (R< means this), why it isn't done?

    Anyone ever met such a problem? Your thought?


    Here is my stress test script, you may try it on SMP platforms (the more
    number of cpu is , the easier it can be regenerated.).

    ########################################################################

    ####### stresscpus.sh #######

    #!/bin/sh

    for i in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
    do
    ./bugexposure.sh $i &
    done


    ######## bugexposure.sh ###########

    #!/bin/bash

    main(){

    typeset -i CPU=$1
    ./task.sh > /dev/null&
    PID=$!

    if [ `cat /sys/devices/system/cpu/cpu${CPU}/online` = "0" ]; then
    echo "1" > /sys/devices/system/cpu/cpu${CPU}/online
    fi

    MASK=$((1<<${CPU}))

    `taskset -p ${MASK} ${PID} > /dev/null 2>&1`

    echo "0" > /sys/devices/system/cpu/cpu${CPU}/online

    echo "1" > /sys/devices/system/cpu/cpu${CPU}/online

    disown $PID
    kill -9 $PID > /dev/null 2>&1

    #echo "PASS\n"

    }

    typeset -i TEST_CPU=$1
    while true
    do
    main $TEST_CPU
    done


    ###### task.sh ######
    #!/bin/bash

    while :
    do
    NOOP=1
    done

    ########################################################################

    My tracing and analysis info is below:

    # ps aux | grep watchdog
    root 5 0.0 0.0 0 0 ? S< 22:26 0:00 [watchdog/0]
    root 8 0.0 0.0 0 0 ? R< 22:26 0:00 [watchdog/1]
    root 11 0.0 0.0 0 0 ? S< 22:26 0:00 [watchdog/2]
    root 14 0.0 0.0 0 0 ? S< 22:26 0:00 [watchdog/3]
    root 17 0.0 0.0 0 0 ? S< 22:26 0:00 [watchdog/4]
    root 20 0.0 0.0 0 0 ? S< 22:26 0:00 [watchdog/5]
    root 23 0.0 0.0 0 0 ? S< 22:26 0:00 [watchdog/6]
    root 26 0.0 0.0 0 0 ? S< 22:26 0:00 [watchdog/7]
    root 29 0.0 0.0 0 0 ? S< 22:26 0:00 [watchdog/8]
    root 32 0.0 0.0 0 0 ? S< 22:26 0:00 [watchdog/9]
    root 35 0.0 0.0 0 0 ? S< 22:26 0:00 [watchdog/10]
    root 38 0.0 0.0 0 0 ? S< 22:26 0:00 [watchdog/11]
    root 41 0.0 0.0 0 0 ? S< 22:26 0:00 [watchdog/12]
    root 44 0.0 0.0 0 0 ? S< 22:26 0:00 [watchdog/13]
    root 50 0.0 0.0 0 0 ? S< 22:26 0:00 [watchdog/15]
    root 5641 0.0 0.0 61144 708 pts/0 R+ 22:58 0:00 grep watchdog
    #

    # ps aux | grep ksoftirqd
    root 4 0.0 0.0 0 0 ? S< 22:26 0:00 [ksoftirqd/0]
    root 10 0.0 0.0 0 0 ? S< 22:26 0:00 [ksoftirqd/2]
    root 13 0.0 0.0 0 0 ? S< 22:26 0:00 [ksoftirqd/3]
    root 16 0.0 0.0 0 0 ? S< 22:26 0:00 [ksoftirqd/4]
    root 19 0.0 0.0 0 0 ? S< 22:26 0:00 [ksoftirqd/5]
    root 22 0.0 0.0 0 0 ? S< 22:26 0:00 [ksoftirqd/6]
    root 25 0.0 0.0 0 0 ? S< 22:26 0:00 [ksoftirqd/7]
    root 28 0.0 0.0 0 0 ? S< 22:26 0:00 [ksoftirqd/8]
    root 31 0.0 0.0 0 0 ? S< 22:26 0:00 [ksoftirqd/9]
    root 34 0.0 0.0 0 0 ? S< 22:26 0:00 [ksoftirqd/10]
    root 37 0.0 0.0 0 0 ? S< 22:26 0:00 [ksoftirqd/11]
    root 40 0.0 0.0 0 0 ? S< 22:26 0:00 [ksoftirqd/12]
    root 43 0.0 0.0 0 0 ? S< 22:26 0:00 [ksoftirqd/13]
    root 49 0.0 0.0 0 0 ? S< 22:26 0:00 [ksoftirqd/15]
    root 5647 0.0 0.0 61144 712 pts/0 R+ 23:00 0:00 grep ksoftirqd
    #

    #ps aux
    ...
    root 5554 0.0 0.0 0 0 ? S< 22:42 0:00 [kstopmachine]
    root 5556 0.0 0.0 0 0 ? Z< 22:42 0:00 [kstopmachine] <defunct>
    root 5557 0.0 0.0 0 0 ? Z< 22:42 0:00 [kstopmachine] <defunct>
    root 5558 0.0 0.0 0 0 ? Z< 22:42 0:00 [kstopmachine] <defunct>
    root 5559 0.0 0.0 0 0 ? Z< 22:42 0:00 [kstopmachine] <defunct>
    root 5560 0.0 0.0 0 0 ? Z< 22:42 0:00 [kstopmachine] <defunct>
    root 5561 0.0 0.0 0 0 ? Z< 22:42 0:00 [kstopmachine] <defunct>
    root 5562 0.0 0.0 0 0 ? Z< 22:42 0:00 [kstopmachine] <defunct>
    root 5563 0.0 0.0 0 0 ? Z< 22:42 0:00 [kstopmachine] <defunct>
    root 5564 0.0 0.0 0 0 ? Z< 22:42 0:00 [kstopmachine] <defunct>
    root 5565 0.0 0.0 0 0 ? Z< 22:42 0:00 [kstopmachine] <defunct>
    root 5566 0.0 0.0 0 0 ? Z< 22:42 0:00 [kstopmachine] <defunct>
    root 5567 0.0 0.0 0 0 ? Z< 22:42 0:00 [kstopmachine] <defunct>
    root 5568 0.0 0.0 0 0 ? Z< 22:42 0:00 [kstopmachine] <defunct>
    root 5569 0.0 0.0 0 0 ? Z< 22:42 0:00 [kstopmachine] <defunct>
    ...


    [watchdog/1] isn't terminated so that [kstopmachine] will waiting for it
    for ever, so that "echo 0 > /sys/devices/syste/cpu/cpu1/online" will hang
    up, so that hotplug spinlock will be holden by it, so that other spinlock
    contenter will wait for locking it for ever, so that...

    These are some softlock detection output:

    process 5471 (task.sh) no longer affine to cpu1
    INFO: task group_balance:51 blocked for more than 120 seconds.
    "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    group_balance D ffff810001011580 0 51 2
    ffff81027e97fdc0 0000000000000046 ffff81027e97fdd0 ffffffff80472b15
    0000000000000000 ffff81027e97d280 ffff81027c8bd300 ffff81027e97d5d0
    0000000080569da0 ffff81027e97d5d0 000000007e97d280 000000010009d366
    Call Trace:
    [<ffffffff80472b15>] thread_return+0x3d/0xa2
    [<ffffffff8023cce5>] lock_timer_base+0x26/0x4b
    [<ffffffff8047338b>] __mutex_lock_slowpath+0x5c/0x93
    [<ffffffff80473221>] mutex_lock+0x1a/0x1e
    [<ffffffff8024f8fd>] get_online_cpus+0x27/0x3e
    [<ffffffff8022fd04>] load_balance_monitor+0x60/0x375
    [<ffffffff8022fca4>] load_balance_monitor+0x0/0x375
    [<ffffffff80245ff6>] kthread+0x47/0x75
    [<ffffffff802301f0>] schedule_tail+0x28/0x5c
    [<ffffffff8020cc78>] child_rip+0xa/0x12
    [<ffffffff80245faf>] kthread+0x0/0x75
    [<ffffffff8020cc6e>] child_rip+0x0/0x12

    INFO: task bugexposure.sh:5462 blocked for more than 120 seconds.
    "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    bugexposure.s D ffff8100010e1580 0 5462 1
    ffff81026a981c58 0000000000000082 ffff81026a981c68 ffffffff80472b15
    00000000000004d6 ffff81027c83c920 ffff81027e8bd680 ffff81027c83cc70
    0000000d7e8bc4c0 ffff81027c83cc70 0000000d00000002 000000010009d809
    Call Trace:
    [<ffffffff80472b15>] thread_return+0x3d/0xa2
    [<ffffffff80314b25>] __next_cpu+0x19/0x28
    [<ffffffff8022a5d4>] enqueue_rt_entity+0x4e/0xb1
    [<ffffffff80472f34>] schedule_timeout+0x1e/0xad
    [<ffffffff802280bf>] enqueue_task+0x4d/0x58
    [<ffffffff80472ccf>] wait_for_common+0xd5/0x118
    [<ffffffff8022c31b>] default_wake_function+0x0/0xe
    [<ffffffff80245c93>] kthread_stop+0x58/0x79
    [<ffffffff8046fdcf>] cpu_callback+0x189/0x197
    [<ffffffff8046f59e>] cpu_callback+0x1cf/0x274
    [<ffffffff8026c07e>] writeback_set_ratelimit+0x19/0x68
    [<ffffffff8047658b>] notifier_call_chain+0x29/0x4c
    [<ffffffff8024f74d>] _cpu_down+0x1d6/0x2aa
    [<ffffffff8024f845>] cpu_down+0x24/0x31
    [<ffffffff8038fcf4>] store_online+0x29/0x67
    [<ffffffff802d23e1>] sysfs_write_file+0xd2/0x110
    [<ffffffff8028db49>] vfs_write+0xad/0x136
    [<ffffffff8028e086>] sys_write+0x45/0x6e
    [<ffffffff8020bfd9>] tracesys+0xdc/0xe1





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