diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h index 4121ffd0faf8..9e9ff13adda8 100644 --- a/include/net/sch_generic.h +++ b/include/net/sch_generic.h @@ -173,37 +173,17 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc) if (spin_trylock(&qdisc->seqlock)) return true; - /* Paired with smp_mb__after_atomic() to make sure - * STATE_MISSED checking is synchronized with clearing - * in pfifo_fast_dequeue(). + /* No need to insist if the MISSED flag was already set. + * Note that test_and_set_bit() also gives us memory ordering + * guarantees wrt potential earlier enqueue() and below + * spin_trylock(), both of which are necessary to prevent races */ - smp_mb__before_atomic(); - - /* If the MISSED flag is set, it means other thread has - * set the MISSED flag before second spin_trylock(), so - * we can return false here to avoid multi cpus doing - * the set_bit() and second spin_trylock() concurrently. - */ - if (test_bit(__QDISC_STATE_MISSED, &qdisc->state)) + if (test_and_set_bit(__QDISC_STATE_MISSED, &qdisc->state)) return false; - /* Set the MISSED flag before the second spin_trylock(), - * if the second spin_trylock() return false, it means - * other cpu holding the lock will do dequeuing for us - * or it will see the MISSED flag set after releasing - * lock and reschedule the net_tx_action() to do the - * dequeuing. - */ - set_bit(__QDISC_STATE_MISSED, &qdisc->state); - - /* spin_trylock() only has load-acquire semantic, so use - * smp_mb__after_atomic() to ensure STATE_MISSED is set - * before doing the second spin_trylock(). - */ - smp_mb__after_atomic(); - - /* Retry again in case other CPU may not see the new flag - * after it releases the lock at the end of qdisc_run_end(). + /* Try to take the lock again to make sure that we will either + * grab it or the CPU that still has it will see MISSED set + * when testing it in qdisc_run_end() */ return spin_trylock(&qdisc->seqlock); } else if (qdisc_is_running(qdisc)) {