netdev - Re: [PATCH v5 net 6/6] net/sched: qdisc_destroy() old ingress and clsact Qdiscs before grafting

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Date: Tue, 30 May 2023 02:11:16 -0700
From: Peilin Ye <yepeilin.cs@...il.com>
To: Vlad Buslov <vladbu@...dia.com>
Cc: Jamal Hadi Salim <jhs@...atatu.com>, Jakub Kicinski <kuba@...nel.org>,
	Pedro Tammela <pctammela@...atatu.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>,
	Peilin Ye <peilin.ye@...edance.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	John Fastabend <john.fastabend@...il.com>,
	Hillf Danton <hdanton@...a.com>, netdev@...r.kernel.org,
	Cong Wang <cong.wang@...edance.com>
Subject: Re: [PATCH v5 net 6/6] net/sched: qdisc_destroy() old ingress and
 clsact Qdiscs before grafting

On Mon, May 29, 2023 at 02:50:26PM +0300, Vlad Buslov wrote:
> After looking very carefully at the code I think I know what the issue
> might be:
>
>    Task 1 graft Qdisc   Task 2 new filter
>            +                    +
>            |                    |
>            v                    v
>         rtnl_lock()       take  q->refcnt
>            +                    +
>            |                    |
>            v                    v
> Spin while q->refcnt!=1   Block on rtnl_lock() indefinitely due to -EAGAIN
>
> This will cause a real deadlock with the proposed patch. I'll try to
> come up with a better approach. Sorry for not seeing it earlier.

Thanks a lot for pointing this out!  The reproducers add flower filters to
ingress Qdiscs so I didn't think of rtnl_lock()'ed filter requests...

On Mon, May 29, 2023 at 03:58:50PM +0300, Vlad Buslov wrote:
> - Account for such cls_api behavior in sch_api by dropping and
>   re-tacking the lock before replaying. This actually seems to be quite
>   straightforward since 'replay' functionality that we are reusing for
>   this is designed for similar behavior - it releases rtnl lock before
>   loading a sch module, takes the lock again and safely replays the
>   function by re-obtaining all the necessary data.

Yes, I've tested this using that reproducer Pedro posted.

On Mon, May 29, 2023 at 03:58:50PM +0300, Vlad Buslov wrote:
> If livelock with concurrent filters insertion is an issue, then it can
> be remedied by setting a new Qdisc->flags bit
> "DELETED-REJECT-NEW-FILTERS" and checking for it together with
> QDISC_CLASS_OPS_DOIT_UNLOCKED in order to force any concurrent filter
> insertion coming after the flag is set to synchronize on rtnl lock.

Thanks for the suggestion!  I'll try this approach.

Currently QDISC_CLASS_OPS_DOIT_UNLOCKED is checked after taking a refcnt of
the "being-deleted" Qdisc.  I'll try forcing "late" requests (that arrive
later than Qdisc is flagged as being-deleted) sync on RTNL lock without
(before) taking the Qdisc refcnt (otherwise I think Task 1 will replay for
even longer?).

Thanks,
Peilin Ye