netdev - Re: [ovs-dev] [PATCH net v2] openvswitch: fix a possible deadlock and lockdep warning

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Date:	Thu, 27 Mar 2014 14:44:34 -0300
From:	Flavio Leitner <fbl@...hat.com>
To:	Jesse Gross <jesse@...ira.com>
Cc:	Pravin Shelar <pshelar@...ira.com>,
	netdev <netdev@...r.kernel.org>,
	openvswitch <dev@...nvswitch.org>
Subject: Re: [ovs-dev] [PATCH net v2] openvswitch: fix a possible deadlock
 and lockdep warning

On Thu, Mar 27, 2014 at 10:37:32AM -0700, Jesse Gross wrote:
> On Thu, Mar 27, 2014 at 10:33 AM, Flavio Leitner <fbl@...hat.com> wrote:
> > On Thu, Mar 27, 2014 at 10:19:23AM -0700, Pravin Shelar wrote:
> >> On Thu, Mar 27, 2014 at 7:05 AM, Flavio Leitner <fbl@...hat.com> wrote:
> >> > There are two problematic situations.
> >> >
> >> > A deadlock can happen when is_percpu is false because it can get
> >> > interrupted while holding the spinlock. Then it executes
> >> > ovs_flow_stats_update() in softirq context which tries to get
> >> > the same lock.
> >> >
> >> > The second sitation is that when is_percpu is true, the code
> >> > correctly disables BH but only for the local CPU, so the
> >> > following can happen when locking the remote CPU without
> >> > disabling BH:
> >> >
> >> >        CPU#0                            CPU#1
> >> >   ovs_flow_stats_get()
> >> >    stats_read()
> >> >  +->spin_lock remote CPU#1        ovs_flow_stats_get()
> >> >  |  <interrupted>                  stats_read()
> >> >  |  ...                       +-->  spin_lock remote CPU#0
> >> >  |                            |     <interrupted>
> >> >  |  ovs_flow_stats_update()   |     ...
> >> >  |   spin_lock local CPU#0 <--+     ovs_flow_stats_update()
> >> >  +---------------------------------- spin_lock local CPU#1
> >> >
> >> > This patch disables BH for both cases fixing the deadlocks.
> >>
> >> This bug is already fixed in OVS.
> >
> > Could you point me to the commit? I am not finding anything
> > recent.
> 
> This is the commit:
> 
> commit 9d73c9cac76ba557fdac4a89c1b7eafe132b85a3
> Author: Pravin B Shelar <pshelar@...ira.com>
> Date:   Tue Dec 17 15:43:30 2013 -0800
> 
>     datapath: Fix deadlock during stats update.
> 
> I thought that I had sent it in the most recent batch of changes for
> net but it looks like I missed it.

That commit is incomplete. Look at the scenario #2 which I explain
why it is needed to disable bh for all cpus and not just local ones.

fbl
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