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    SubjectRe: Using sparse to catch invalid RCU dereferences?
    On Tue, Apr 08, 2008 at 12:04:16AM +0200, Johannes Berg wrote:
    > Hi,
    >
    > Just a thought, I haven't tried this yet because I'm not entirely sure
    > it's actually correct. I was just thinking it should be possible to
    > introduce something like
    >
    > #define __rcu __attribute__((address_space(3)))
    >
    > (for sparse only, of course) and then be able to say
    >
    > struct myfoo *foo __rcu;
    >
    > and sparse would warn on
    >
    > struct myfoo *bar = foo;
    >
    > but not on
    >
    > struct myfoo *bar = rcu_dereference(foo);

    Ah, "address_space" is a sparse-ism, no wonder I couldn't find it in
    the gcc docs...

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

    > by way of using __force inside rcu_dereference(), rcu_assign_pointer()
    > etc.
    >
    > Would this be feasible? Or should one actually use __bitwise/__force to
    > also get the warning when assigning between two variables both marked
    > __rcu?

    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;

    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.

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

    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!!!

    Thanx, Paul


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