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    SubjectRe: [tip:sched/core] sched: Fix ancient race in do_exit()
    On 01/28, Linus Torvalds wrote:
    > On Sat, Jan 28, 2012 at 4:03 AM, tip-bot for Yasunori Goto
    > <> wrote:
    > >
    > > sched: Fix ancient race in do_exit()
    > Ugh.
    > It would be much nicer to just clear the rwsem waiter->task thing
    > *after* waking the task up, which would avoid this race entirely,
    > afaik.

    How? The problem is that wake_up_process(tsk) sees this task in
    TASK_UNINTERRUPTIBLE state (the first "p->state & state" check in
    try_to_wake_up), but then this task changes its state to TASK_DEAD
    without schedule() and ttwu() does s/TASK_DEAD/TASK_RUNNING/.

    IOW, the task doing

    current->state = TASK_A;
    current->state = TASK_B;

    can be woken up by try_to_wake_up(TASK_A), despite the fact it
    sleeps in TASK_B. do_exit() is only "special" because it is not
    easy to handle the spurious wakeup.

    > Tell me, why wouldn't that work? rwsem_down_failed_common() does
    > /* wait to be given the lock */
    > for (;;) {
    > if (!waiter.task)
    > break;
    > ...
    > so then we wouldn't need the task refcount crap in rwsem either etc,
    > and we'd get rid of all races with wakeup.
    > I wonder why we're clearing that whole waiter->task so early.

    I must have missed something. I can't understand how this can help,
    and "clear the rwsem waiter->task thing *after* waking" looks
    obviously wrong. If we do this, then we can miss the "!!waiter.task"
    condition. The loop above actually does


    if (!waiter.task)

    waiter->task = NULL;

    can happen right after set_task_state().


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      Last update: 2012-01-29 17:17    [W:0.021 / U:0.964 seconds]
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