Re: [PATCH -tip] kprobes: Lock rcu_read_lock() while searching kprobe

[Ingo Molnar <mingo@kernel.org>]

* Paul E. McKenney <paulmck@kernel.org> wrote:

> >  * This list-traversal primitive may safely run concurrently with
> >  * the _rcu list-mutation primitives such as hlist_add_head_rcu()
> >  * as long as the traversal is guarded by rcu_read_lock().
> >  */
> > #define hlist_for_each_entry_rcu(pos, head, member, cond...)            \
> > 
> > is actively harmful. Why is it there?
> 
> For cases where common code might be invoked both from the reader
> (with RCU protection) and from the updater (protected by some
> lock).  This common code can then use the optional argument to
> hlist_for_each_entry_rcu() to truthfully tell lockdep that it might be
> called with either form of protection in place.
> 
> This also combines with the __rcu tag used to mark RCU-protected
> pointers, in which case sparse complains when a non-RCU API is applied
> to these pointers, to get back to your earlier question about use of
> hlist_for_each_entry_rcu() within the update-side lock.
> 
> But what are you seeing as actively harmful about all of this?
> What should we be doing instead?

Yeah, so basically in the write-locked path hlist_for_each_entry() 
generates (slightly) more efficient code than hlist_for_each_entry_rcu(), 
correct?

Also, the principle of passing warning flags around is problematic - but 
I can see the point in this specific case.

Thanks,

	Ingo