net: sched: fixed barrier to prevent skbuff sticking in qdisc backlog

net: sched: fixed barrier to prevent skbuff sticking in qdisc backlog

In qdisc_run_begin(), smp_mb__before_atomic() used before test_bit()
does not provide any ordering guarantee as test_bit() is not an atomic
operation. This, added to the fact that the spin_trylock() call at
the beginning of qdisc_run_begin() does not guarantee acquire
semantics if it does not grab the lock, makes it possible for the
following statement :

if (test_bit(__QDISC_STATE_MISSED, &qdisc->state))

to be executed before an enqueue operation called before
qdisc_run_begin().

As a result the following race can happen :

           CPU 1                             CPU 2

      qdisc_run_begin()               qdisc_run_begin() /* true */
        set(MISSED)                            .
      /* returns false */                      .
          .                            /* sees MISSED = 1 */
          .                            /* so qdisc not empty */
          .                            __qdisc_run()
          .                                    .
          .                              pfifo_fast_dequeue()
 ----> /* may be done here */                  .
|         .                                clear(MISSED)
|         .                                    .
|         .                                smp_mb __after_atomic();
|         .                                    .
|         .                                /* recheck the queue */
|         .                                /* nothing => exit   */
|   enqueue(skb1)
|         .
|   qdisc_run_begin()
|         .
|     spin_trylock() /* fail */
|         .
|     smp_mb__before_atomic() /* not enough */
|         .
 ---- if (test_bit(MISSED))
        return false;   /* exit */

In the above scenario, CPU 1 and CPU 2 both try to grab the
qdisc->seqlock at the same time. Only CPU 2 succeeds and enters the
bypass code path, where it emits its skb then calls __qdisc_run().

CPU1 fails, sets MISSED and goes down the traditionnal enqueue() +
dequeue() code path. But when executing qdisc_run_begin() for the
second time, after enqueuing its skbuff, it sees the MISSED bit still
set (by itself) and consequently chooses to exit early without setting
it again nor trying to grab the spinlock again.

Meanwhile CPU2 has seen MISSED = 1, cleared it, checked the queue
and found it empty, so it returned.

At the end of the sequence, we end up with skb1 enqueued in the
backlog, both CPUs out of __dev_xmit_skb(), the MISSED bit not set,
and no __netif_schedule() called made. skb1 will now linger in the
qdisc until somebody later performs a full __qdisc_run(). Associated
to the bypass capacity of the qdisc, and the ability of the TCP layer
to avoid resending packets which it knows are still in the qdisc, this
can lead to serious traffic "holes" in a TCP connection.

We fix this by replacing the smp_mb__before_atomic() / test_bit() /
set_bit() / smp_mb__after_atomic() sequence inside qdisc_run_begin()
by a single test_and_set_bit() call, which is more concise and
enforces the needed memory barriers.

Fixes: 89837eb4b246 ("net: sched: add barrier to ensure correct ordering for lockless qdisc")
Signed-off-by: Vincent Ray <vray@kalrayinc.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20220526001746.2437669-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>