* [PATCH net] net: sched: add barrier to ensure correct ordering for lockless qdisc
@ 2021-06-16 12:25 Yunsheng Lin
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From: Yunsheng Lin @ 2021-06-16 12:25 UTC (permalink / raw)
To: davem, kuba
Cc: olteanv, ast, daniel, andriin, edumazet, weiwan, cong.wang,
ap420073, netdev, linux-kernel, linuxarm, mkl, linux-can, jhs,
xiyou.wangcong, jiri, andrii, kafai, songliubraving, yhs,
john.fastabend, kpsingh, bpf, jonas.bonn, pabeni, mzhivich,
johunt, albcamus, kehuan.feng, a.fatoum, atenart,
alexander.duyck, hdanton, jgross, JKosina, mkubecek, bjorn,
alobakin
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@huawei.com>
---
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
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