Re: [PATCH net v8 1/3] net: sched: fix packet stuck problem for lockless qdisc

[Cong Wang]

On Fri, May 14, 2021 at 7:25 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
>
> On 2021/5/15 8:17, Jakub Kicinski wrote:
> > On Fri, 14 May 2021 16:57:29 -0700 Cong Wang wrote:
> >> On Fri, May 14, 2021 at 4:39 PM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
> >>>
> >>> On Fri, 14 May 2021 16:36:16 -0700 Cong Wang wrote:
> >>  [...]
> >>>>
> >>>> We have test_and_clear_bit() which is atomic, test_bit()+clear_bit()
> >>>> is not.
> >>>
> >>> It doesn't have to be atomic, right? I asked to split the test because
> >>> test_and_clear is a locked op on x86, test by itself is not.
> >>
> >> It depends on whether you expect the code under the true condition
> >> to run once or multiple times, something like:
> >>
> >> if (test_bit()) {
> >>   clear_bit();
> >>   // this code may run multiple times
> >> }
> >>
> >> With the atomic test_and_clear_bit(), it only runs once:
> >>
> >> if (test_and_clear_bit()) {
> >>   // this code runs once
> >> }
>
> I am not sure if the above really matter when the test and clear
> does not need to be atomic.
>
> In order for the above to happens, the MISSED has to set between
> test and clear, right?

Nope, see the following:

// MISSED bit is already set
CPU0                            CPU1
if (test_bit(MISSED) ( //true
                                if (test_bit(MISSED)) { // also true
        clear_bit(MISSED);
        do_something();
                                        clear_bit(MISSED);
                                        do_something();
                                }
}

Now do_something() is called twice instead of once. This may or may
not be a problem, hence I asked this question.

>
> >>
> >> This is why __netif_schedule() uses test_and_set_bit() instead of
> >> test_bit()+set_bit().
>
> I think test_and_set_bit() is needed in __netif_schedule() mainly
> because STATE_SCHED is also used to indicate if the qdisc is in
> sd->output_queue, so it has to be atomic.

If you replace the "do_something()" above with __netif_reschedule(),
this is exactly my point. An entry can not be inserted twice into a
list, hence it should never be called twice like above.

Thanks.