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

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Message-ID: <20200123171025.GB4484@workstation-portable>
Date:   Thu, 23 Jan 2020 22:40:25 +0530
From:   Amol Grover <frextrite@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        Joel Fernandes <joel@...lfernandes.org>,
        Madhuparna Bhowmik <madhuparnabhowmik04@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH] bpf: devmap: Pass lockdep expression to RCU lists

On Thu, Jan 23, 2020 at 02:42:03PM +0100, Toke Høiland-Jørgensen wrote:
> Jesper Dangaard Brouer <brouer@...hat.com> writes:
> 
> > On Thu, 23 Jan 2020 17:34:38 +0530
> > Amol Grover <frextrite@...il.com> 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
>