From: Cong Wang <xiyou.wangcong@gmail.com>
To: Vlad Buslov <vladbu@mellanox.com>
Cc: Linux Kernel Network Developers <netdev@vger.kernel.org>,
Jamal Hadi Salim <jhs@mojatatu.com>,
Jiri Pirko <jiri@resnulli.us>, David Miller <davem@davemloft.net>
Subject: Re: [PATCH net-next 01/12] net: sched: flower: don't check for rtnl on head dereference
Date: Wed, 20 Feb 2019 14:33:15 -0800 [thread overview]
Message-ID: <CAM_iQpVSXVbLWcYDo8GHXn=aOPao=XufF5m=GhKAp9BhbJ=E9w@mail.gmail.com> (raw)
In-Reply-To: <vbf8sycqfn8.fsf@mellanox.com>
On Tue, Feb 19, 2019 at 1:46 AM Vlad Buslov <vladbu@mellanox.com> wrote:
>
>
> On Mon 18 Feb 2019 at 19:08, Cong Wang <xiyou.wangcong@gmail.com> wrote:
> > On Wed, Feb 13, 2019 at 11:47 PM Vlad Buslov <vladbu@mellanox.com> wrote:
> >>
> >> Flower classifier only changes root pointer during init and destroy. Cls
> >> API implements reference counting for tcf_proto, so there is no danger of
> >> concurrent access to tp when it is being destroyed, even without protection
> >> provided by rtnl lock.
> >
> > How about atomicity? Refcnt doesn't guarantee atomicity, how do
> > you make sure two concurrent modifications are atomic?
>
> In order to guarantee atomicity I lock shared flower classifier data
> structures with tp->lock in following patches.
Sure, I meant the atomicity of the _whole_ change, as you know
the TC filters are stored in hierarchical structures: a block, a chain,
a tp struct, some type-specific data structure like a hash table.
Locking tp only solves a partial of the atomicity here. Are you
going to restart the whole change from top down to the bottom?
>
> >
> >
> >>
> >> Implement new function fl_head_dereference() to dereference tp->root
> >> without checking for rtnl lock. Use it in all flower function that obtain
> >> head pointer instead of rtnl_dereference().
> >>
> >
> > So what lock protects RCU writers after this patch?
>
> I explained it in comment for fl_head_dereference(), but should have
> copied this information to changelog as well:
> Flower classifier only changes root pointer during init and destroy.
> Cls API implements reference counting for tcf_proto, so there is no
> danger of concurrent access to tp when it is being destroyed, even
> without protection provided by rtnl lock.
So you are saying an RCU pointer is okay to deference without
any lock eve without RCU read lock, right?
>
> In initial version of this change I used tp->lock to protect tp->root
> access and verified it with lockdep, but during internal review Jiri
> noted that this is not needed in current flower implementation.
Let's see what you have on top of your own branch
unlocked_flower_cong_1:
1458 static int fl_change(struct net *net, struct sk_buff *in_skb,
1459 struct tcf_proto *tp, unsigned long base,
1460 u32 handle, struct nlattr **tca,
1461 void **arg, bool ovr, bool rtnl_held,
1462 struct netlink_ext_ack *extack)
1463 {
1464 struct cls_fl_head *head = fl_head_dereference(tp);
At the point of line 1464, there is no lock taken, tp->lock is taken
after it, block->lock or chain lock is already unlocked before ->change().
So, what protects this RCU structure? According to RCU, it must be
either RCU read lock or some writer lock. I see none here. :(
What am I missing?
next prev parent reply other threads:[~2019-02-20 22:33 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-14 7:47 [PATCH net-next 00/12] Refactor flower classifier to remove dependency on rtnl lock Vlad Buslov
2019-02-14 7:47 ` [PATCH net-next 01/12] net: sched: flower: don't check for rtnl on head dereference Vlad Buslov
2019-02-18 19:08 ` Cong Wang
2019-02-19 9:45 ` Vlad Buslov
2019-02-20 22:33 ` Cong Wang [this message]
2019-02-21 17:45 ` Vlad Buslov
2019-02-22 19:32 ` Cong Wang
2019-02-25 16:11 ` Vlad Buslov
2019-02-25 22:39 ` Cong Wang
2019-02-26 14:57 ` Vlad Buslov
2019-02-28 0:49 ` Cong Wang
2019-02-28 18:35 ` Vlad Buslov
2019-03-02 0:51 ` Cong Wang
2019-02-14 7:47 ` [PATCH net-next 02/12] net: sched: flower: refactor fl_change Vlad Buslov
2019-02-14 20:34 ` Stefano Brivio
2019-02-15 10:38 ` Vlad Buslov
2019-02-15 10:47 ` Stefano Brivio
2019-02-15 16:25 ` Vlad Buslov
2019-02-18 18:20 ` Stefano Brivio
2019-02-14 7:47 ` [PATCH net-next 03/12] net: sched: flower: introduce reference counting for filters Vlad Buslov
2019-02-14 20:34 ` Stefano Brivio
2019-02-15 11:22 ` Vlad Buslov
2019-02-15 12:32 ` Stefano Brivio
2019-02-14 7:47 ` [PATCH net-next 04/12] net: sched: flower: track filter deletion with flag Vlad Buslov
2019-02-14 20:49 ` Stefano Brivio
2019-02-15 15:54 ` Vlad Buslov
2019-02-14 7:47 ` [PATCH net-next 05/12] net: sched: flower: add reference counter to flower mask Vlad Buslov
2019-02-14 7:47 ` [PATCH net-next 06/12] net: sched: flower: handle concurrent mask insertion Vlad Buslov
2019-02-15 22:46 ` Stefano Brivio
2019-02-14 7:47 ` [PATCH net-next 07/12] net: sched: flower: protect masks list with spinlock Vlad Buslov
2019-02-14 7:47 ` [PATCH net-next 08/12] net: sched: flower: handle concurrent filter insertion in fl_change Vlad Buslov
2019-02-14 7:47 ` [PATCH net-next 09/12] net: sched: flower: handle concurrent tcf proto deletion Vlad Buslov
2019-02-18 20:47 ` Cong Wang
2019-02-19 14:08 ` Vlad Buslov
2019-02-14 7:47 ` [PATCH net-next 10/12] net: sched: flower: protect flower classifier state with spinlock Vlad Buslov
2019-02-14 7:47 ` [PATCH net-next 11/12] net: sched: flower: track rtnl lock state Vlad Buslov
2019-02-15 22:46 ` Stefano Brivio
2019-02-18 9:35 ` Vlad Buslov
2019-02-14 7:47 ` [PATCH net-next 12/12] net: sched: flower: set unlocked flag for flower proto ops Vlad Buslov
2019-02-18 19:27 ` Cong Wang
2019-02-19 10:15 ` Vlad Buslov
2019-02-20 22:36 ` Cong Wang
2019-02-18 19:15 ` [PATCH net-next 00/12] Refactor flower classifier to remove dependency on rtnl lock Cong Wang
2019-02-19 10:00 ` Vlad Buslov
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