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Date:   Thu, 23 Jan 2020 14:37:25 +0100
From:   Jesper Dangaard Brouer <brouer@redhat.com>
To:     Amol Grover <frextrite@gmail.com>
Cc:     brouer@redhat.com, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        "David S . Miller" <davem@davemloft.net>,
        Jakub Kicinski <jakub.kicinski@netronome.com>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andriin@fb.com>, netdev@vger.kernel.org,
        bpf@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        Joel Fernandes <joel@joelfernandes.org>,
        Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>,
        "Paul E . McKenney" <paulmck@kernel.org>
Subject: Re: [PATCH] bpf: devmap: Pass lockdep expression to RCU lists
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On Thu, 23 Jan 2020 17:34:38 +0530
Amol Grover <frextrite@gmail.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?

If it is then it looks fine to me:

Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
 
> Hence, add corresponding lockdep expression to silence false-positive
> lockdep warnings, and harden RCU lists.
> 
> Signed-off-by: Amol Grover <frextrite@gmail.com>
> ---
>  kernel/bpf/devmap.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> index 3d3d61b5985b..b4b6b77f309c 100644
> --- a/kernel/bpf/devmap.c
> +++ b/kernel/bpf/devmap.c
> @@ -293,7 +293,8 @@ struct bpf_dtab_netdev *__dev_map_hash_lookup_elem(struct bpf_map *map, u32 key)
>  	struct hlist_head *head = dev_map_index_hash(dtab, key);
>  	struct bpf_dtab_netdev *dev;
>  
> -	hlist_for_each_entry_rcu(dev, head, index_hlist)
> +	hlist_for_each_entry_rcu(dev, head, index_hlist,
> +				 lockdep_is_held(&dtab->index_lock))
>  		if (dev->idx == key)
>  			return dev;
>  

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer