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?
>
> .
>
