On Sun, Jul 22 2018, Paul E. McKenney wrote:
>
> One issue is that the ->func pointer can legitimately be NULL while on
> RCU's callback lists.  This happens when someone invokes kfree_rcu()
> with the rcu_head structure at the beginning of the enclosing structure.
> I could add an offset to avoid this, or perhaps the kmalloc() folks
> could be persuaded Rao Shoaib's patch moving kfree_rcu() handling to
> the slab allocators, so that RCU only ever sees function pointers in
> the ->func field.
>
> Either way, this should be hidden behind an API to allow adjustments
> to be made if needed.  Maybe something like is_after_call_rcu()?
> This would (for example) allow debug-object checks to be used to catch
> check-after-free bugs.
>
> Would something of that sort work for you?

Yes, if you could provide an is_after_call_rcu() API, that would
perfectly suit my use-case.
Thanks,
NeilBrown