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 -- To unsubscribe from this list: send the line "unsubscribe linux-sparse" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
