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); 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?
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