On Mon, Jul 23 2018, Paul E. McKenney wrote:

> On Mon, Jul 23, 2018 at 09:13:43AM +1000, NeilBrown wrote:
>> 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.
>
> After beating my head against the object-debug code a bit, I have to ask
> if it would be OK for you if the is_after_call_rcu() API also takes the
> function that was passed to RCU.

Sure.  It feels a bit clumsy, but I can see it could be easier to make
robust.
So yes: I'm fine with pass the same function and rcu_head to both
call_rcu() and is_after_call_rcu().  Actually, when I say it like that,
it seems less clumsy :-)

Thanks,
NeilBrown