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

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Message-Id: <20210315.164151.1093629330365238718.davem@redhat.com>
Date:   Mon, 15 Mar 2021 16:41:51 -0700 (PDT)
From:   David Miller <davem@...hat.com>
To:     linyunsheng@...wei.com
Cc:     kuba@...nel.org, jhs@...atatu.com, xiyou.wangcong@...il.com,
        jiri@...nulli.us, netdev@...r.kernel.org,
        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

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.