netdev - Re: [PATCH net] tipc: block BH before using dst

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Date:   Fri, 22 May 2020 14:44:37 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jon Maloy <jmaloy@...hat.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Xin Long <lucien.xin@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        syzbot <syzkaller@...glegroups.com>,
        tipc-discussion@...ts.sourceforge.net
Subject: Re: [PATCH net] tipc: block BH before using dst_cache

On Fri, May 22, 2020 at 2:37 PM Jon Maloy <jmaloy@...hat.com> wrote:
>
>
>
> On 5/22/20 4:10 PM, Eric Dumazet wrote:
> >
> > On 5/22/20 12:47 PM, Jon Maloy wrote:
> >>
> >> On 5/22/20 11:57 AM, Eric Dumazet wrote:
> >>> On 5/22/20 8:01 AM, Jon Maloy wrote:
> >>>> On 5/22/20 2:18 AM, Xin Long wrote:
> >>>>> On Fri, May 22, 2020 at 1:55 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >>>>>> Resend to the list in non HTML form
> >>>>>>
> >>>>>>
> >>>>>> On Thu, May 21, 2020 at 10:53 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >>>>>>> On Thu, May 21, 2020 at 10:50 PM Xin Long <lucien.xin@...il.com> wrote:
> >>>>>>>> On Fri, May 22, 2020 at 2:30 AM Eric Dumazet <edumazet@...gle.com> wrote:
> >>>>>>>>> dst_cache_get() documents it must be used with BH disabled.
> >>>>>>>> Interesting, I thought under rcu_read_lock() is enough, which calls
> >>>>>>>> preempt_disable().
> >>>>>>> rcu_read_lock() does not disable BH, never.
> >>>>>>>
> >>>>>>> And rcu_read_lock() does not necessarily disable preemption.
> >>>>> Then I need to think again if it's really worth using dst_cache here.
> >>>>>
> >>>>> Also add tipc-discussion and Jon to CC list.
> >>>> The suggested solution will affect all bearers, not only UDP, so it is not a good.
> >>>> Is there anything preventing us from disabling preemtion inside the scope of the rcu lock?
> >>>>
> >>>> ///jon
> >>>>
> >>> BH is disabled any way few nano seconds later, disabling it a bit earlier wont make any difference.
> >> The point is that if we only disable inside tipc_udp_xmit() (the function pointer call) the change will only affect the UDP bearer, where dst_cache is used.
> >> The corresponding calls for the Ethernet and Infiniband bearers don't use dst_cache, and don't need this disabling. So it does makes a difference.
> >>
> > I honestly do not understand your concern, this makes no sense to me.
> >
> > I have disabled BH _right_ before the dst_cache_get(cache) call, so has no effect if the dst_cache is not used, this should be obvious.
> Forget my comment. I thought we were discussing to Tetsuo Handa's
> original patch, and missed that you had posted your own.
> I have no problems with this one.
>

Ah, this now makes sense, I was not aware of Tetsuo patch.

You are absolutely right, Tetsuo Handa's patch is wrong.

Thanks