From: Jakub Kicinski <kuba@kernel.org>
To: Yunsheng Lin <yunshenglin0825@gmail.com>
Cc: Yunsheng Lin <linyunsheng@huawei.com>,
davem@davemloft.net, olteanv@gmail.com, ast@kernel.org,
daniel@iogearbox.net, andriin@fb.com, edumazet@google.com,
weiwan@google.com, cong.wang@bytedance.com, ap420073@gmail.com,
netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
linuxarm@openeuler.org, mkl@pengutronix.de,
linux-can@vger.kernel.org, jhs@mojatatu.com,
xiyou.wangcong@gmail.com, jiri@resnulli.us, andrii@kernel.org,
kafai@fb.com, songliubraving@fb.com, yhs@fb.com,
john.fastabend@gmail.com, kpsingh@kernel.org,
bpf@vger.kernel.org, jonas.bonn@netrounds.com, pabeni@redhat.com,
mzhivich@akamai.com, johunt@akamai.com, albcamus@gmail.com,
kehuan.feng@gmail.com, a.fatoum@pengutronix.de,
atenart@kernel.org, alexander.duyck@gmail.com, hdanton@sina.com,
jgross@suse.com, JKosina@suse.com, mkubecek@suse.cz,
bjorn@kernel.org, alobakin@pm.me
Subject: Re: [PATCH net v2] net: sched: add barrier to ensure correct ordering for lockless qdisc
Date: Mon, 21 Jun 2021 16:29:48 -0700 [thread overview]
Message-ID: <20210621162948.179f0753@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com> (raw)
In-Reply-To: <20210619103009.GA1530@ip-172-31-30-86.us-east-2.compute.internal>
On Sat, 19 Jun 2021 10:30:09 +0000 Yunsheng Lin wrote:
> When debugging pointed to the misordering between STATE_MISSED
> setting/clearing and STATE_MISSED checking, only _after_atomic()
> was added first, and it did not fix the misordering problem,
> when both _before_atomic() and _after_atomic() were added, the
> misordering problem disappeared.
>
> I suppose _before_atomic() matters because the STATE_MISSED
> setting and the lock rechecking is only done when first check of
> STATE_MISSED returns false. _before_atomic() is used to make sure
> the first check returns correct result, if it does not return the
> correct result, then we may have misordering problem too.
>
> cpu0 cpu1
> clear MISSED
> _after_atomic()
> dequeue
> enqueue
> first trylock() #false
> MISSED check #*true* ?
>
> As above, even cpu1 has a _after_atomic() between clearing
> STATE_MISSED and dequeuing, we might stiil need a barrier to
> prevent cpu0 doing speculative MISSED checking before cpu1
> clearing MISSED?
>
> And the implicit load-acquire barrier contained in the first
> trylock() does not seems to prevent the above case too.
>
> And there is no load-acquire barrier in pfifo_fast_dequeue()
> too, which possibly make the above case more likely to happen.
Ah, you're right. The test_bit() was not in the patch context,
I forgot it's there... Both barriers are indeed needed.
prev parent reply other threads:[~2021-06-21 23:29 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-17 1:04 [PATCH net v2] net: sched: add barrier to ensure correct ordering for lockless qdisc Yunsheng Lin
2021-06-19 0:30 ` Jakub Kicinski
2021-06-19 0:38 ` Jakub Kicinski
2021-06-19 10:30 ` Yunsheng Lin
2021-06-21 23:29 ` Jakub Kicinski [this message]
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