Re: cpu_hotplug vs oom_notify_list: possible circular locking dependency detected

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On Thu, Jul 16, 2015 at 02:01:56PM -0700, David Rientjes wrote:
> On Wed, 15 Jul 2015, Paul E. McKenney wrote:
> 
> > On Tue, Jul 14, 2015 at 04:48:24PM -0700, David Rientjes wrote:
> > > On Tue, 14 Jul 2015, Paul E. McKenney wrote:
> > > 
> > > > commit a1992f2f3b8e174d740a8f764d0d51344bed2eed
> > > > Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > > Date:   Tue Jul 14 16:24:14 2015 -0700
> > > > 
> > > >     rcu: Don't disable CPU hotplug during OOM notifiers
> > > >     
> > > >     RCU's rcu_oom_notify() disables CPU hotplug in order to stabilize the
> > > >     list of online CPUs, which it traverses.  However, this is completely
> > > >     pointless because smp_call_function_single() will quietly fail if invoked
> > > >     on an offline CPU.  Because the count of requests is incremented in the
> > > >     rcu_oom_notify_cpu() function that is remotely invoked, everything works
> > > >     nicely even in the face of concurrent CPU-hotplug operations.
> > > >     
> > > >     Furthermore, in recent kernels, invoking get_online_cpus() from an OOM
> > > >     notifier can result in deadlock.  This commit therefore removes the
> > > >     call to get_online_cpus() and put_online_cpus() from rcu_oom_notify().
> > > >     
> > > >     Reported-by: Marcin Ślusarz <marcin.slusarz@xxxxxxxxx>
> > > >     Reported-by: David Rientjes <rientjes@xxxxxxxxxx>
> > > >     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > 
> > > Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> > 
> > Thank you!
> > 
> > Any news on whether or not it solves the problem?
> > 
> 
> Marcin, is your lockdep violation reproducible?  If so, does this patch 
> fix it?

I finally found enough time today to test it. I can reproduce it without
the above patch and can't with. So:
Tested-by: Marcin Ślusarz <marcin.slusarz@xxxxxxxxx>

Thanks,
Marcin

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