[ovs-dev] [PATCH net v2] openvswitch: fix a possible deadlock and lockdep warning
Flavio Leitner
fbl at redhat.com
Thu Mar 27 17:44:34 UTC 2014
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 at redhat.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 at redhat.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 at nicira.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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