On Tue, Mar 16, 2021 at 1:35 AM Yunsheng Lin linyunsheng@huawei.com wrote:
On 2021/3/16 2:53, Jakub Kicinski wrote:
On Mon, 15 Mar 2021 11:10:18 +0800 Yunsheng Lin wrote:
@@ -606,6 +623,11 @@ static const u8 prio2band[TC_PRIO_MAX + 1] = { */ struct pfifo_fast_priv { struct skb_array q[PFIFO_FAST_BANDS];
- /* protect against data race between enqueue/dequeue and
* qdisc->empty setting*/- spinlock_t lock;
};
static inline struct skb_array *band2list(struct pfifo_fast_priv *priv, @@ -623,7 +645,10 @@ static int pfifo_fast_enqueue(struct sk_buff *skb, struct Qdisc *qdisc, unsigned int pkt_len = qdisc_pkt_len(skb); int err;
- err = skb_array_produce(q, skb);
spin_lock(&priv->lock);
err = __ptr_ring_produce(&q->ring, skb);
WRITE_ONCE(qdisc->empty, false);
spin_unlock(&priv->lock);
if (unlikely(err)) { if (qdisc_is_percpu_stats(qdisc))
@@ -642,6 +667,7 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc) struct sk_buff *skb = NULL; int band;
- spin_lock(&priv->lock); for (band = 0; band < PFIFO_FAST_BANDS && !skb; band++) { struct skb_array *q = band2list(priv, band);
@@ -655,6 +681,7 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc) } else { WRITE_ONCE(qdisc->empty, true); }
spin_unlock(&priv->lock);
return skb;
}
I thought pfifo was supposed to be "lockless" and this change re-introduces a lock between producer and consumer, no?
Yes, the lock breaks the "lockless" of the lockless qdisc for now I do not how to solve the below data race locklessly:
CPU1: CPU2: dequeue skb . . . . enqueue skb . . . WRITE_ONCE(qdisc->empty, false); . . . .WRITE_ONCE(qdisc->empty, true);
Maybe it is time to fully document/explain how this can possibly work.
lockless qdisc used concurrently by multiple cpus, using WRITE_ONCE() and READ_ONCE() ?
Just say no to this.
If the above happens, the qdisc->empty is true even if the qdisc has some skb, which may cuase out of order or packet stuck problem.
It seems we may need to update ptr_ring' status(empty or not) while enqueuing/dequeuing atomically in the ptr_ring implementation.
Any better idea?