netdev - Re: [PATCH bpf-next 03/17] dev: add rcu_read_lock_bh_held() as a valid check when getting a RCU dev ref

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Date:   Fri, 11 Jun 2021 01:05:38 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Hangbin Liu <liuhangbin@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH bpf-next 03/17] dev: add rcu_read_lock_bh_held() as a
 valid check when getting a RCU dev ref

Martin KaFai Lau <kafai@...com> writes:

> On Wed, Jun 09, 2021 at 12:33:12PM +0200, Toke Høiland-Jørgensen wrote:
>> Some of the XDP helpers (in particular, xdp_do_redirect()) will get a
>> struct net_device reference using dev_get_by_index_rcu(). These are called
>> from a NAPI poll context, which means the RCU reference liveness is ensured
>> by local_bh_disable(). Add rcu_read_lock_bh_held() as a condition to the
>> RCU list traversal in dev_get_by_index_rcu() so lockdep understands that
>> the dereferences are safe from *both* an rcu_read_lock() *and* with
>> local_bh_disable().
>> 
>> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
>> ---
>>  net/core/dev.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index febb23708184..a499c5ffe4a5 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -1002,7 +1002,7 @@ struct net_device *dev_get_by_index_rcu(struct net *net, int ifindex)
>>  	struct net_device *dev;
>>  	struct hlist_head *head = dev_index_hash(net, ifindex);
>>  
>> -	hlist_for_each_entry_rcu(dev, head, index_hlist)
>> +	hlist_for_each_entry_rcu(dev, head, index_hlist, rcu_read_lock_bh_held())
> Is it needed?  hlist_for_each_entry_rcu() checks for
> rcu_read_lock_any_held().  Did lockdep complain?

Ah, yes, I think you're right. I totally missed that
rcu_read_lock_any_held() includes a '!preemptible()' check at the end.
I'll drop this patch, then!

-Toke