From: Yunsheng Lin <linyunsheng@huawei.com>
To: <davem@davemloft.net>, <kuba@kernel.org>
Cc: <olteanv@gmail.com>, <ast@kernel.org>, <daniel@iogearbox.net>,
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<bjorn@kernel.org>, <alobakin@pm.me>
Subject: [PATCH net v2] net: sched: add barrier to ensure correct ordering for lockless qdisc
Date: Thu, 17 Jun 2021 09:04:14 +0800 [thread overview]
Message-ID: <1623891854-57416-1-git-send-email-linyunsheng@huawei.com> (raw)
The spin_trylock() was 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.
Fixes: a90c57f2cedd ("net: sched: fix packet stuck problem for lockless qdisc")
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
---
V2: add the missing Fixes tag.
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
next reply other threads:[~2021-06-17 1:04 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-17 1:04 Yunsheng Lin [this message]
2021-06-19 0:30 ` [PATCH net v2] net: sched: add barrier to ensure correct ordering for lockless qdisc Jakub Kicinski
2021-06-19 0:38 ` Jakub Kicinski
2021-06-19 10:30 ` Yunsheng Lin
2021-06-21 23:29 ` Jakub Kicinski
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