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

On Fri, 14 May 2021 16:57:29 -0700 Cong Wang wrote:

On Fri, May 14, 2021 at 4:39 PM Jakub Kicinski [off-list ref] wrote:

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On Fri, 14 May 2021 16:36:16 -0700 Cong Wang wrote:  

 [...]  

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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.