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    SubjectRe: [PATCH] sys_getppid oopses on debug kernel (v2)
    >>Your patch doesn't cure the problem.
    >>rcu_read_lock just disables preemtion and rcu_dereference
    >>introduces memory barrier. _None_ of this _prevents_
    >>another CPU from freeing old real_parent in parallel with your dereference.
    >
    >
    > How so? Note that release_task() doesn't call put_task_struct(), it does
    > call_rcu(&p->rcu, delayed_put_task_struct) instead. When delayed_put_task_struct()
    > is called, all CPUs must see the new value of ->real_parent (otherwise
    > RCU is just broken). If CPU sees the old value of ->real_parent, rcu_read_lock()
    > protects us from delayed_put_task_struct() on another CPU.
    >
    > Ok, I think this is the same "classic" pattern as:
    >
    > old = global_ptr;
    > global_ptr = new;
    > call_rcu(..free_old...);
    > vs
    > rcu_read_lock();
    > use(global_ptr);
    > rcu_read_unlock();
    >
    > Do you agree?
    Agree :)
    Sorry, I didn't notice that task structs are freed now with RCU :)))
    In this case your patch really helps the problem. Care to prepare it?

    Thanks,
    Kirill
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