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

From: Cong Wang
Date: Tue Mar 16 2021 - 14:42:31 EST

On Mon, Mar 15, 2021 at 2:29 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
> 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.

What's the benefit here? Since qdisc->q.lock is also per-qdisc,
so there is no actual contention to take it when we already acquire
q->seqlock, right?

Also, is ->seqlock supposed to be used for protecting skb_bad_txq
etc.? From my understanding, it was introduced merely for replacing
__QDISC_STATE_RUNNING. If you want to extend it, you probably
have to rename it too.