On Fri, Apr 11, 2008 at 08:18:42PM +0200, Peter Zijlstra wrote: > On Tue, 2008-04-08 at 08:52 -0700, Paul E. McKenney wrote: > > 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. > > We could start by annotating those as well, for example: > > __rcu spinlock_t tree_lock; > > Then we would know that when tree lock is held the data structure is > stable and we can ommit the rcu_*() functions. Good point! Though IIRC there are are cases where we are updating one RCU-protected data structure while in an RCU read-side critical section with respect to another RCU-protected data structure. But it would probably best to start as you say rather than trying to classify different RCU uses. :-) 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
