netdev - Re: [Patch net-next] ipv4: get rid of ip_ra

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Message-ID: <CAM_iQpXWvw+v7wFnSr=BMOfFPwEYAyfO26ZGspXeUaGsWAPCKQ@mail.gmail.com>
Date:   Fri, 28 Apr 2017 09:54:58 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [Patch net-next] ipv4: get rid of ip_ra_lock

On Thu, Apr 27, 2017 at 4:54 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2017-04-27 at 16:46 -0700, Cong Wang wrote:
>> On Thu, Apr 27, 2017 at 5:46 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > On Wed, 2017-04-26 at 13:55 -0700, Cong Wang wrote:
>> >> After commit 1215e51edad1 ("ipv4: fix a deadlock in ip_ra_control")
>> >> we always take RTNL lock for ip_ra_control() which is the only place
>> >> we update the list ip_ra_chain, so the ip_ra_lock is no longer needed,
>> >> we just need to disable BH there.
>> >>
>> >> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
>> >> ---
>> >
>> > Looks great, but reading again this code, I believe we do not need to
>> > disable BH at all ?
>> >
>>
>> Hmm, if we don't disable BH here, a reader in BH could jump in and
>> break this critical section? Or that is fine for RCU?
>
> It should be fine for RCU.
>
> The spinlock (or mutex if this is RTNL) is protecting writers among
> themselves. Here it should run in process context, with no specific
> rules to disable preemption, hard or soft irqs.
>
> The reader(s) do not care of how writer(s) enforce their mutual
> protection, and if writer(s) disable hard or soft irqs.

Fair enough! This refreshes my understanding of RCU.

I will send V2. The ASSERT_RTNL() is unnecessary too, as
we already have one in rcu_dereference_protected().

Thanks!