Re: [PATCH] bpf: devmap: Pass lockdep expression to RCU lists

[Amol Grover]

On Thu, Jan 23, 2020 at 02:42:03PM +0100, Toke HÃiland-JÃrgensen wrote:
> Jesper Dangaard Brouer <brouer@xxxxxxxxxx> writes:
> 
> > On Thu, 23 Jan 2020 17:34:38 +0530
> > Amol Grover <frextrite@xxxxxxxxx> wrote:
> >
> >> head is traversed using hlist_for_each_entry_rcu outside an
> >> RCU read-side critical section but under the protection
> >> of dtab->index_lock.
> >
> > We do hold the lock in update and delete cases, but not in the lookup
> > cases.  Is it then still okay to add the lockdep_is_held() annotation?
> 
> I concluded 'yes' from the comment on hlist_for_each_entry_rcu():
> 
> The lockdep condition gets passed to this:
> 
> #define __list_check_rcu(dummy, cond, extra...)				\
> 	({								\
> 	check_arg_count_one(extra);					\
> 	RCU_LOCKDEP_WARN(!cond && !rcu_read_lock_any_held(),		\
> 			 "RCU-list traversed in non-reader section!");	\
> 	 })
> 
> 
> so that seems fine :)
> 

Yes, adding a lockdep expression will be okay. This is because an
implicit check is done to check if list_for_each_entry_rcu() is
traversed under RCU read-side critical section. In case the traversal is
outside RCU read-side critical section, the lockdep expression makes
sure the traversal is done under the mentioned lock.

Thanks
Amol

> -Toke
>