Re: Using sparse to catch invalid RCU dereferences?

[Peter Zijlstra <peterz@infradead.org>]

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.