netdev - Re: [PATCH net v2] net: sched: add barrier to ensure correct ordering for lockless qdisc

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Message-ID: <20210618173837.0131edc3@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Fri, 18 Jun 2021 17:38:37 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     <davem@...emloft.net>, <olteanv@...il.com>, <ast@...nel.org>,
        <daniel@...earbox.net>, <andriin@...com>, <edumazet@...gle.com>,
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        <ap420073@...il.com>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linuxarm@...neuler.org>,
        <mkl@...gutronix.de>, <linux-can@...r.kernel.org>,
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        <alobakin@...me>
Subject: Re: [PATCH net v2] net: sched: add barrier to ensure correct
 ordering for lockless qdisc

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@...wei.com>  
> 
> Acked-by: Jakub Kicinski <kuba@...nel.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.