[PATCH net] net: sched: add barrier to ensure correct ordering for lockless qdisc

[Yunsheng Lin]

The spin_trylock() is assumed to contain the implicit
barrier needed to ensure the correct ordering between
STATE_MISSED setting/clearing and STATE_MISSED checking
in commit a90c57f2cedd ("net: sched: fix packet stuck
problem for lockless qdisc").

But it turns out that spin_trylock() only has load-acquire
semantic, for strongly-ordered system(like x86), the compiler
barrier implicitly contained in spin_trylock() seems enough
to ensure the correct ordering. But for weakly-orderly system
(like arm64), the store-release semantic is needed to ensure
the correct ordering as clear_bit() and test_bit() is store
operation, see queued_spin_lock().

So add the explicit barrier to ensure the correct ordering
for the above case.

Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
---
The above ordering issue can easily cause out of order packet
problem when testing lockless qdisc bypass patchset [1] with
two iperf threads and one netdev queue in arm64 system.

1. https://lkml.org/lkml/2021/6/2/1417
---
 include/net/sch_generic.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 1e62551..5771030 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -163,6 +163,12 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
 		if (spin_trylock(&qdisc->seqlock))
 			goto nolock_empty;
 
+		/* Paired with smp_mb__after_atomic() to make sure
+		 * STATE_MISSED checking is synchronized with clearing
+		 * in pfifo_fast_dequeue().
+		 */
+		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
@@ -180,6 +186,12 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
 		 */
 		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().
 		 */
-- 
2.7.4