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

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Date:   Thu, 23 Jan 2020 14:42:03 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Amol Grover <frextrite@...il.com>
Cc:     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

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 :)

-Toke