Re: [PATCH net v2] net: sched: fix packet stuck problem for lockless qdisc

[Yunsheng Lin]

On 2021/3/25 3:20, Cong Wang wrote:
> On Tue, Mar 23, 2021 at 7:24 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
>> @@ -176,8 +207,23 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
>>  static inline void qdisc_run_end(struct Qdisc *qdisc)
>>  {
>>         write_seqcount_end(&qdisc->running);
>> -       if (qdisc->flags & TCQ_F_NOLOCK)
>> +       if (qdisc->flags & TCQ_F_NOLOCK) {
>>                 spin_unlock(&qdisc->seqlock);
>> +
>> +               /* qdisc_run_end() is protected by RCU lock, and
>> +                * qdisc reset will do a synchronize_net() after
>> +                * setting __QDISC_STATE_DEACTIVATED, so testing
>> +                * the below two bits separately should be fine.
> 
> Hmm, why synchronize_net() after setting this bit is fine? It could
> still be flipped right after you test RESCHEDULE bit.

That depends on when it will be fliped again.

As I see:
1. __QDISC_STATE_DEACTIVATED is set during dev_deactivate() process,
   which should also wait for all process related to "test_bit(
   __QDISC_STATE_NEED_RESCHEDULE, &q->state)" to finish by calling
   synchronize_net() and checking some_qdisc_is_busy().

2. it is cleared during dev_activate() process.

And dev_deactivate() and dev_activate() is protected by RTNL lock, or
serialized by linkwatch.

> 
> 
>> +                * For qdisc_run() in net_tx_action() case, we
>> +                * really should provide rcu protection explicitly
>> +                * for document purposes or PREEMPT_RCU.
>> +                */
>> +               if (unlikely(test_bit(__QDISC_STATE_NEED_RESCHEDULE,
>> +                                     &qdisc->state) &&
>> +                            !test_bit(__QDISC_STATE_DEACTIVATED,
>> +                                      &qdisc->state)))
> 
> Why do you want to test __QDISC_STATE_DEACTIVATED bit at all?
> dev_deactivate_many() will wait for those scheduled but being
> deactivated, so what's the problem of scheduling it even with this bit?

The problem I tried to fix is:

  CPU0(calling dev_deactivate)   CPU1(calling qdisc_run_end)   CPU2(calling tx_atcion)
             .                       __netif_schedule()                   .
             .                     set __QDISC_STATE_SCHED                .
             .                                .                           .
clear __QDISC_STATE_DEACTIVATED               .                           .
     synchronize_net()                        .                           .
             .                                .                           .
             .                                .              clear __QDISC_STATE_SCHED
             .                                .                           .
 some_qdisc_is_busy() return false            .                           .
             .                                .                           .
             .                                .                      qdisc_run()

some_qdisc_is_busy() checks if the qdisc is busy by checking __QDISC_STATE_SCHED
and spin_is_locked(&qdisc->seqlock) for lockless qdisc, and some_qdisc_is_busy()
return false for CPU0 because CPU2 has cleared the __QDISC_STATE_SCHED and has not
taken the qdisc->seqlock yet, qdisc is clearly still busy when qdisc_run() is run
by CPU2 later.

So you are right, testing __QDISC_STATE_DEACTIVATED does not completely solve
the above data race, and there are __netif_schedule() called by dev_requeue_skb()
and __qdisc_run() too, which need the same fixing.

So will remove the __QDISC_STATE_DEACTIVATED testing for this patch first, and
deal with it later.

> 
> Thanks.
> 
> .
>