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Date:   Mon, 15 Mar 2021 16:41:51 -0700 (PDT)
Message-Id: <20210315.164151.1093629330365238718.davem@redhat.com>
To:     linyunsheng@huawei.com
Cc:     kuba@kernel.org, jhs@mojatatu.com, xiyou.wangcong@gmail.com,
        jiri@resnulli.us, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, linuxarm@openeuler.org
Subject: Re: [PATCH net-next] net: sched: remove unnecessay lock protection
 for skb_bad_txq/gso_skb
From:   David Miller <davem@redhat.com>
In-Reply-To: <1615800610-34700-1-git-send-email-linyunsheng@huawei.com>
References: <1615800610-34700-1-git-send-email-linyunsheng@huawei.com>
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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.