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

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Date:	Thu, 27 Mar 2014 13:01:30 -0700
From:	Jesse Gross <jesse@...ira.com>
To:	Pravin Shelar <pshelar@...ira.com>
Cc:	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 11:40 AM, Pravin Shelar <pshelar@...ira.com> wrote:
> On Thu, Mar 27, 2014 at 11:32 AM, Jesse Gross <jesse@...ira.com> wrote:
>> On Thu, Mar 27, 2014 at 10:44 AM, Flavio Leitner <fbl@...hat.com> wrote:
>>> 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.
>>
>> OK, I understand the second problem now. OVS master (which I am
>> currently working to cross-port to net-next) uses a different strategy
>> that also always disables bottom halves for a different reason. Since
>> I forgot to send the original patch, maybe we can just apply this one
>> to net instead and use the new stuff directly everywhere else.
>>
>
> The ovs patch fixes locking issue.
>
> local cpu check is optimization for better latency which can be
> significant on large SMP system. I think we should fix lockdep rather
> than adding latency for ovs packet processing. But all this is going
> to change with NUMA stats anyways, so I do not have any problem
> pushing this new patch.

I think there is actually a possible deadlock here, not just a lockdep
warning. If you look at Flavio's diagram, it's a rather complicated
series of events that would be necessary but it seems theoretically
possible.
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