netdev - RE: [PATCH v20 net-next 1/6] sched: Struct definition and parsing of dualpi2 qdisc

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Message-ID:
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Date: Tue, 1 Jul 2025 05:41:53 +0000
From: "Chia-Yu Chang (Nokia)" <chia-yu.chang@...ia-bell-labs.com>
To: Jakub Kicinski <kuba@...nel.org>
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Subject: RE: [PATCH v20 net-next 1/6] sched: Struct definition and parsing of
 dualpi2 qdisc

> -----Original Message-----
> From: Jakub Kicinski <kuba@...nel.org> 
> Sent: Tuesday, July 1, 2025 1:43 AM
> To: Chia-Yu Chang (Nokia) <chia-yu.chang@...ia-bell-labs.com>
> Cc: alok.a.tiwari@...cle.com; pctammela@...atatu.com; horms@...nel.org; donald.hunter@...il.com; xandfury@...il.com; netdev@...r.kernel.org; dave.taht@...il.com; pabeni@...hat.com; jhs@...atatu.com; stephen@...workplumber.org; xiyou.wangcong@...il.com; jiri@...nulli.us; davem@...emloft.net; edumazet@...gle.com; andrew+netdev@...n.ch; ast@...erby.net; liuhangbin@...il.com; shuah@...nel.org; linux-kselftest@...r.kernel.org; ij@...nel.org; ncardwell@...gle.com; Koen De Schepper (Nokia) <koen.de_schepper@...ia-bell-labs.com>; g.white@...lelabs.com; ingemar.s.johansson@...csson.com; mirja.kuehlewind@...csson.com; cheshire@...le.com; rs.ietf@....at; Jason_Livingood@...cast.com; vidhi_goel@...le.com
> Subject: Re: [PATCH v20 net-next 1/6] sched: Struct definition and parsing of dualpi2 qdisc
> 
> 
> CAUTION: This is an external email. Please be very careful when clicking links or opening attachments. See the URL nok.it/ext for additional information.
> 
> 
> 
> On Mon, 30 Jun 2025 17:04:56 +0000 Chia-Yu Chang (Nokia) wrote:
> > > > This still needs 2 WRITE_ONCE even "step_thresh" (NLA_U32) and 
> > > > "step_in_packtes" (NLA_FLAG) are replaced with "step_pkt_thresh"
> > > > (NLA_U32) and "step_time_thresh" (NLA_U32) - which was proposed in 
> > > > my another email.
> > >
> > > If you don't understand the question - ask for clarifications :/
> >
> > You are right.
> >
> > Could you elaborate on the orignal comment "And the block under which 
> > I'm responding is performing two dependent writes, one to
> > ->step_in_packets and the other to ->step_thresh a change which is
> > definitely not atomic.."?
> >
> > I don't see we access the same atomic variable multiple times in a 
> > single expression, the 2 WRITE_ONCE() are in different expressions.
> >
> > And, in the last WRITE_ONCE(), what we access are local variables:
> > "step_pkt" "step_th", will it create problem?
> 
> Not really a problem, but what I'm saying is that I don't understand why all the writes are sprinkled with WRITE_ONCE(). You take
> 
>         sch_tree_lock(sch);
> 
> to block data path and the control path is under rtnl_lock. So why the WRITE_ONCE()? WRITE_ONCE() is used to annotate writes which can be read concurrently without holding relevant locks.

This follows series https://lore.kernel.org/netdev/20240415132054.3822230-1-edumazet@google.com/ to use READ_ONCE in dual2_dump() and WRITE_ONCE in dualpi2_change.

And because of one previous chnage request (https://lore.kernel.org/all/26f2f366-aa14-4879-978a-58d46f9d83a4@redhat.com/) to split single dualpi2.c into 3 patches, so you can find them in the 2nd patch of this series.

What I can do is either move these WRITE_ONCE also to the next patch or merge these patches.

I would prefer the first approach to make it still 3 patches, any other suggestion?