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

[Jakub Kicinski]

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
> }
> 
> This is why __netif_schedule() uses test_and_set_bit() instead of
> test_bit()+set_bit().

Thanks, makes sense, so hopefully the MISSED-was-set case is not common
and we can depend on __netif_schedule() to DTRT, avoiding the atomic op
in the common case.