Re: Using sparse to catch invalid RCU dereferences?

[Johannes Berg <johannes@sipsolutions.net>]

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

Yeah. It's currently used for __user and __iomem. Using it for rcu
wouldn't be quite the way it was intended, I think, but hey :)

> 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;

Yeah, all of those would lead to sparse warnings. Are we willing to
change all that code?

> 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.

I don't think I would be that person :)

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

Right. Those too would lead to problems, unless we change that code to
use those (or other) macros.

> 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!!!

I'll play with it a bit if I get around, was just reviewing some RCU
usage and had the feeling that it should be possible to automate.

johannes

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