Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH net-next] net: sched: remove unnecessay lock protection
 for skb_bad_txq/gso_skb
To:     Cong Wang <xiyou.wangcong@gmail.com>,
        David Miller <davem@redhat.com>
CC:     Jakub Kicinski <kuba@kernel.org>,
        Jamal Hadi Salim <jhs@mojatatu.com>,
        Jiri Pirko <jiri@resnulli.us>,
        Linux Kernel Network Developers <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, <linuxarm@openeuler.org>
References: <1615800610-34700-1-git-send-email-linyunsheng@huawei.com>
 <20210315.164151.1093629330365238718.davem@redhat.com>
 <CAM_iQpWPSouO-JP4xHFqOLM8H4Rn5ucF68sa_EK5hUWSYw8feA@mail.gmail.com>
From:   Yunsheng Lin <linyunsheng@huawei.com>
Message-ID: <a8874cd1-b7c4-5307-db46-8906c0949e12@huawei.com>
Date:   Wed, 17 Mar 2021 08:50:17 +0800
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101
 Thunderbird/52.2.0
MIME-Version: 1.0
In-Reply-To: <CAM_iQpWPSouO-JP4xHFqOLM8H4Rn5ucF68sa_EK5hUWSYw8feA@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 2021/3/17 2:43, Cong Wang wrote:
> On Mon, Mar 15, 2021 at 4:42 PM David Miller <davem@redhat.com> wrote:
>>
>> From: Yunsheng Lin <linyunsheng@huawei.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@huawei.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.
> 
> .
>