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    SubjectRe: while_each_thread() under rcu_read_lock() is broken?
    On 06/21, Roland McGrath wrote:
    >
    > > Paul, Roland, do you see any problems from the correctness pov,
    > > or a better fix for now?
    > >
    > > Perhaps it also makes sense to keep the old variant renamed to
    > > while_each_thread_locked(), I dunno.
    >
    > Did we verify that only de_thread() can create the situation where a
    > while_each_thread-style loop without either lock can be confused?

    I think yes, this is is the only case.

    I mean, while_each_thread(group_leader, t). If g != group_leader, then
    the lockless while_each_thread() has problems with the plain exit(g).

    Afaics. The more I think about this, the more I feel confused ;)

    But if we start from ->group_leader, then while_each_thread() must
    stop eventually. Otherwise we should assume that the dead (unhashed)
    tasks can create the circular list, obviously this is not possible.

    > If
    > that's so, then just changing it to avoid the situation seems like it
    > would be less invasive overall.

    How? We should change ->group_leader uner write_lock_irq(tasklist),
    synchronize_rcu() is not an option. We can't do call_rcu(release_task),
    we can't take tasklist for writing in the softirq context. But even
    if we could, this can't help in fact or I missed something.

    Oleg.



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