On 2021/3/17 2:41, Cong Wang wrote: > On Mon, Mar 15, 2021 at 2:29 AM Yunsheng Lin <linyunsh...@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?
Yes, there is no actual contention to take qdisc->q.lock while q->seqlock is acquired, but a cleanup or minor optimization. > > 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. How about just using qdisc->q.lock for lockless qdisc too and remove dqisc->seqlock completely? > > Thanks. > > . >
