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    Subject[BUG] TASK_DEAD task is able to be woken up in special condition

    Hello

    I found TASK_DEAD task is able to be woken up in special condition.
    I would like to report this bug. Please check it.

    Here is the sequence how it occurs.

    ----------------------------------+-----------------------------
    |
    CPU A | CPU B
    ----------------------------------+-----------------------------
    TASK A calls exit()....

    do_exit()

    exit_mm()
    down_read(mm->mmap_sem);

    rwsem_down_failed_common()

    set TASK_UNINTERRUPTIBLE
    set waiter.task <= task A
    list_add to sem->wait_list
    :
    raw_spin_unlock_irq()
    (I/O interruption occured)

    __rwsem_do_wake(mmap_sem)

    list_del(&waiter->list);
    waiter->task = NULL
    wake_up_process(task A)
    try_to_wake_up()
    (task is still
    TASK_UNINTERRUPTIBLE)
    p->on_rq is still 1.)

    ttwu_do_wakeup()
    (*A)
    :
    (I/O interruption handler finished)

    if (!waiter.task)
    schedule() is not called
    due to waiter.task is NULL.

    tsk->state = TASK_RUNNING

    :
    check_preempt_curr();
    :
    task->state = TASK_DEAD
    (*B)
    <--- set TASK_RUNNING (*C)



    schedule()
    (exit task is running again)
    BUG_ON() is called!
    --------------------------------------------------------


    You probably think that execution time between (*A) and (*B) is very short,
    because the interruption is disabled, and setting TASK_RUNNING at (*C)
    must be executed before setting TASK_DEAD.


    HOWEVER, if SMI is interrupted between (*A) and (*B),
    (*C) is able to execute AFTER setting TASK_DEAD!
    Then, exited task is scheduled again, and BUG_ON() is called....

    This is very bad senario.
    But, I suppose this phenomenon is able to occur on a guest system of
    virtual machine too.

    Please fix it.

    I suppose task->pi_lock should be held when task->state is changed to
    TASK_DEAD like the following patch (not tested yet).
    Because try_to_wake_up() hold it before checking task state.


    Thanks,

    ----
    Signed-off-by: Yasunori Goto <y-goto@jp.fujitsu.com>

    ---
    kernel/exit.c | 3 +++
    1 file changed, 3 insertions(+)

    Index: linux-3.2-rc4/kernel/exit.c
    ===================================================================
    --- linux-3.2-rc4.orig/kernel/exit.c
    +++ linux-3.2-rc4/kernel/exit.c
    @@ -1038,8 +1038,11 @@ NORET_TYPE void do_exit(long code)

    preempt_disable();
    exit_rcu();
    +
    + spin_lock(&tsk->pi_lock, flags);
    /* causes final put_task_struct in finish_task_switch(). */
    tsk->state = TASK_DEAD;
    + spin_unlock(&tsk->pi_lock, flags);
    schedule();
    BUG();
    /* Avoid "noreturn function does return". */



    --
    Yasunori Goto




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