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    SubjectRe: Using sparse to catch invalid RCU dereferences?

    > So the address_space attribute says what the pointer points to rather
    > than where the pointer resides, correct?

    Yeah. It's currently used for __user and __iomem. Using it for rcu
    wouldn't be quite the way it was intended, I think, but hey :)

    > It might be. There are a number of places where it is legal to access
    > RCU-protected pointers directly, and all of these would need to be
    > changed. For example, in the example above, one could do:
    > foo = NULL;

    Yeah, all of those would lead to sparse warnings. Are we willing to
    change all that code?

    > I recently tried to modify rcu_assign_pointer() to issue the memory
    > memory barrier only when the pointer was non-NULL, but this ended badly.
    > Probably because I am not the greatest gcc expert around... We ended
    > up having to define an rcu_assign_index() to handle the possibility of
    > assigning a zero-value array index, but my attempts to do type-checking
    > backfired, and I eventually gave it up. Again, someone a bit more clued
    > in to gcc than I am could probably pull it off.

    I don't think I would be that person :)

    > In addition, it is legal to omit rcu_dereference() and rcu_assign_pointer()
    > when holding the update-side lock.

    Right. Those too would lead to problems, unless we change that code to
    use those (or other) macros.

    > So I very much like this approach in general, but it will require some
    > care to implement. I would be very happy to review and comment!!!

    I'll play with it a bit if I get around, was just reviewing some RCU
    usage and had the feeling that it should be possible to automate.

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