From: Jakub Kicinski <kuba@kernel.org>
To: Yunsheng Lin <linyunsheng@huawei.com>
Cc: <davem@davemloft.net>, <olteanv@gmail.com>, <ast@kernel.org>,
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Subject: Re: [PATCH net v2] net: sched: add barrier to ensure correct ordering for lockless qdisc
Date: Fri, 18 Jun 2021 17:38:37 -0700 [thread overview]
Message-ID: <20210618173837.0131edc3@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com> (raw)
In-Reply-To: <20210618173047.68db0b81@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
On Fri, 18 Jun 2021 17:30:47 -0700 Jakub Kicinski wrote:
> On Thu, 17 Jun 2021 09:04:14 +0800 Yunsheng Lin wrote:
> > 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>
>
> Acked-by: Jakub Kicinski <kuba@kernel.org>
Actually.. do we really need the _before_atomic() barrier?
I'd think we only need to make sure we re-check the lock
after we set the bit, ordering of the first check doesn't
matter.
next prev parent reply other threads:[~2021-06-19 0:38 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 [this message]
2021-06-19 10:30 ` Yunsheng Lin
2021-06-21 23:29 ` Jakub Kicinski
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