On Mon, Mar 15, 2021 at 2:29 AM Yunsheng Lin linyunsheng@huawei.com 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.
Thanks.