linux-kernel - Re: [PATCH net-next] net: sched: remove unnecessay lock protection for skb_bad_txq/gso

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Date:   Wed, 17 Mar 2021 08:50:17 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Cong Wang <xiyou.wangcong@...il.com>,
        David Miller <davem@...hat.com>
CC:     Jakub Kicinski <kuba@...nel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, <linuxarm@...neuler.org>
Subject: Re: [PATCH net-next] net: sched: remove unnecessay lock protection
 for skb_bad_txq/gso_skb

On 2021/3/17 2:43, Cong Wang wrote:
> On Mon, Mar 15, 2021 at 4:42 PM David Miller <davem@...hat.com> wrote:
>>
>> From: Yunsheng Lin <linyunsheng@...wei.com>
>> Date: Mon, 15 Mar 2021 17:30:10 +0800
>>
>>> Currently qdisc_lock(q) is taken before enqueuing and dequeuing
>>> for lockless qdisc's skb_bad_txq/gso_skb queue, qdisc->seqlock is
>>> also taken, which can provide the same protection as qdisc_lock(q).
>>>
>>> This patch removes the unnecessay qdisc_lock(q) protection for
>>> lockless qdisc' skb_bad_txq/gso_skb queue.
>>>
>>> And dev_reset_queue() takes the qdisc->seqlock for lockless qdisc
>>> besides taking the qdisc_lock(q) when doing the qdisc reset,
>>> some_qdisc_is_busy() takes both qdisc->seqlock and qdisc_lock(q)
>>> when checking qdisc status. It is unnecessary to take both lock
>>> while the fast path only take one lock, so this patch also changes
>>> it to only take qdisc_lock(q) for locked qdisc, and only take
>>> qdisc->seqlock for lockless qdisc.
>>>
>>> Since qdisc->seqlock is taken for lockless qdisc when calling
>>> qdisc_is_running() in some_qdisc_is_busy(), use qdisc->running
>>> to decide if the lockless qdisc is running.
>>>
>>> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
>>
>> What about other things protected by this lock, such as statistics and qlen?
>>
>> This change looks too risky to me.
> 
> They are per-cpu for pfifo_fast which sets TCQ_F_CPUSTATS too.

Did you mean qdisc_lock(q) are protecting per-cpu stats for
pfifo_fast too?

> 
> Thanks.
> 
> .
>