Messages in this thread | | | Date | Tue, 8 Apr 2008 08:52:59 -0700 | From | "Paul E. McKenney" <> | Subject | Re: Using sparse to catch invalid RCU dereferences? |
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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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