netdev - Re: [Patch net] smc: use RCU version of lower netdev searching

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Message-ID: <ZuUDv8PLR4FHg+oC@pop-os.localdomain>
Date: Fri, 13 Sep 2024 20:32:15 -0700
From: Cong Wang <xiyou.wangcong@...il.com>
To: "D. Wythe" <alibuda@...ux.alibaba.com>
Cc: netdev@...r.kernel.org, Cong Wang <cong.wang@...edance.com>,
	syzbot+c75d1de73d3b8b76272f@...kaller.appspotmail.com,
	Wenjia Zhang <wenjia@...ux.ibm.com>,
	Jan Karcher <jaka@...ux.ibm.com>,
	Tony Lu <tonylu@...ux.alibaba.com>,
	Wen Gu <guwen@...ux.alibaba.com>
Subject: Re: [Patch net] smc: use RCU version of lower netdev searching

On Sat, Sep 14, 2024 at 10:28:15AM +0800, D. Wythe wrote:
> 
> 
> On 9/14/24 8:53 AM, Cong Wang wrote:
> > On Thu, Sep 12, 2024 at 02:20:47PM +0800, D. Wythe wrote:
> > > 
> > > 
> > > On 9/12/24 8:04 AM, Cong Wang wrote:
> > > > From: Cong Wang <cong.wang@...edance.com>
> > > > 
> > > > Both netdev_walk_all_lower_dev() and netdev_lower_get_next() have a
> > > > RCU version, which are netdev_walk_all_lower_dev_rcu() and
> > > > netdev_next_lower_dev_rcu(). Switching to the RCU version would
> > > > eliminate the need for RTL lock, thus could amend the deadlock
> > > > complaints from syzbot. And it could also potentially speed up its
> > > > callers like smc_connect().
> > > > 
> > > > Reported-by: syzbot+c75d1de73d3b8b76272f@...kaller.appspotmail.com
> > > > Closes: https://syzkaller.appspot.com/bug?extid=c75d1de73d3b8b76272f
> > > > Cc: Wenjia Zhang <wenjia@...ux.ibm.com>
> > > > Cc: Jan Karcher <jaka@...ux.ibm.com>
> > > > Cc: "D. Wythe" <alibuda@...ux.alibaba.com>
> > > > Cc: Tony Lu <tonylu@...ux.alibaba.com>
> > > > Cc: Wen Gu <guwen@...ux.alibaba.com>
> > > > Signed-off-by: Cong Wang <cong.wang@...edance.com>
> > > 
> > > 
> > > Haven't looked at your code yet, but the issue you fixed doesn't exist.
> > > The real reason is that we lacks some lockdep annotations for
> > > IPPROTO_SMC.
> > 
> > If you look at the code, it is not about sock lock annotations, it is
> > about RTNL lock which of course has annotations.
> > 
> 
> If so, please explain the deadlock issue mentioned in sysbot and
> how it triggers deadlocks.

Sure, but what questions do you have here? To me, the lockdep output is
self-explained. Please kindly let me know if you have any troubles
understanding it, I am always happy to help.

Thanks.