On 2021/3/16 8:35, Yunsheng Lin 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);
>
> 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?
It seems we can use __ptr_ring_empty() within the qdisc->seqlock protection,
because qdisc->seqlock is clearly served as r->consumer_lock.
>
>>
>> .
>>
> _______________________________________________
> Linuxarm mailing list -- linuxarm@xxxxxxxxxxxxx
> To unsubscribe send an email to linuxarm-leave@xxxxxxxxxxxxx
>
