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    SubjectRe: introduce task_rcu_dereference?
    Damn.

    On 10/22, Oleg Nesterov wrote:
    >
    > +struct task_struct *task_rcu_dereference(struct task_struct **ptask)
    > +{
    > + struct task_struct *task;
    > + struct sighand_struct *sighand;
    > +
    > + task = rcu_dereference(*ptask);
    > + if (!task)
    > + return NULL;
    > +
    > + /* If it fails the check below must fail too */
    > + probe_slab_address(&task->sighand, sighand);
    > + /*
    > + * 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. The caller might want to retry in
    > + * this case.
    > + */
    > + smp_rmb();
    > + if (unlikely(task != ACCESS_ONCE(*ptask)))
    > + return ERR_PTR(-EAGAIN);

    This is not exactly right. task == *ptask can be false positive.

    It can be freed, then resused (so that sighand != NULL can be false
    positive), then freed again, and then reused again as task_struct.

    This is not that bad, we still can safely use this task_struct, but
    the comment should be updated. Plus -EINVAL below can be wrong in
    this case although this minor.

    Yeees, SLAB_DESTTROY_BY_RCU closes this race. Not sure why I'd like
    to avoid it, but I do ;)

    Oleg.



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