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    SubjectRe: [PATCH v3] sched/numa: fix unsafe get_task_struct() in task_numa_assign()
    On 10/20, Oleg Nesterov wrote:
    > Again, perhaps we will need to change the lifetime rules for task_struct
    > anyway, if we have more problems like this. But until then this looks like
    > an overkill to me. Plus rq_curr_if_not_put() looks too subtle, and it is
    > not generic.

    Yes... otoh, perhaps we can do something more generic? Something like

    struct task_struct *xxx(struct task_struct **ptask)
    struct task_struct *task;
    void *sighand;
    task = ACCESS_ONCE(*ptask);
    if (!task)
    return NULL;

    if (probe_kernel_read(&sighand, &task->sighand, sizeof(sighand)))
    goto retry;
    } else {
    sighand = task->sighand;

    if (!sighand)
    return NULL;
    * Pairs with atomic_dec_and_test() in put_task_struct(task).
    * If we have read the freed/reused memory, we must see that
    * the pointer was updated.
    if (task != ACCESS_ONCE(*ptask))
    goto retry;

    return task;

    task_numa_compare() can do cur = xxx(&rc->curr), but this helper can work
    with any "task_struct *" pointer assuming that somehow this pointer is
    cleared or changed before the final put_task_struct().

    What do you think? Peter?


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