netdev - Re: [PATCH stable] net: sch_generic: fix the missing new qdisc assignment bug

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Date:   Tue, 3 Nov 2020 10:46:28 -0500
From:   Vishwanath Pai <vpai@...mai.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>, gregkh@...uxfoundation.org,
        stable@...r.kernel.org
Cc:     Joakim.Tjernlund@...inera.com, xiyou.wangcong@...il.com,
        johunt@...mai.com, jhs@...atatu.com, jiri@...nulli.us,
        davem@...emloft.net, kuba@...nel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linuxarm@...wei.com,
        john.fastabend@...il.com, eric.dumazet@...il.com, dsahern@...il.com
Subject: Re: [PATCH stable] net: sch_generic: fix the missing new qdisc
 assignment bug

On 11/2/20 10:25 PM, Yunsheng Lin wrote:
 > commit 2fb541c862c9 ("net: sch_generic: aviod concurrent reset and 
enqueue op for lockless qdisc")
 >
 > When the above upstream commit is backported to stable kernel,
 > one assignment is missing, which causes two problems reported
 > by Joakim and Vishwanath, see [1] and [2].
 >
 > So add the assignment back to fix it.
 >
 > 1. 
https://urldefense.com/v3/__https://www.spinics.net/lists/netdev/msg693916.html__;!!GjvTz_vk!AqzcoNtwXeDu-vDNRKnOiOWYmi4B-2atZZExjZTvpp2jeJ9asOyQBVUtQyBp$
 > 2. 
https://urldefense.com/v3/__https://www.spinics.net/lists/netdev/msg695131.html__;!!GjvTz_vk!AqzcoNtwXeDu-vDNRKnOiOWYmi4B-2atZZExjZTvpp2jeJ9asOyQBQlaitCQ$
 >
 > Fixes: 749cc0b0c7f3 ("net: sch_generic: aviod concurrent reset and 
enqueue op for lockless qdisc")
 > Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
 > ---
 >  net/sched/sch_generic.c | 3 +++
 >  1 file changed, 3 insertions(+)
 >
 > diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
 > index 0e275e1..6e6147a 100644
 > --- a/net/sched/sch_generic.c
 > +++ b/net/sched/sch_generic.c
 > @@ -1127,10 +1127,13 @@ static void dev_deactivate_queue(struct 
net_device *dev,
 >                   void *_qdisc_default)
 >  {
 >      struct Qdisc *qdisc = rtnl_dereference(dev_queue->qdisc);
 > +    struct Qdisc *qdisc_default = _qdisc_default;
 >
 >      if (qdisc) {
 >          if (!(qdisc->flags & TCQ_F_BUILTIN))
 >              set_bit(__QDISC_STATE_DEACTIVATED, &qdisc->state);
 > +
 > +        rcu_assign_pointer(dev_queue->qdisc, qdisc_default);
 >      }
 >  }
 >

I have tested the patch on v5.4.71 and it fixes our issues.

Tested-by: Vishwanath Pai <vpai@...mai.com>