linux-kernel - Re: cpu_hotplug vs oom_notify_list: possible circular locking dependency detected

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Date:	Mon, 20 Jul 2015 17:48:03 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Marcin Ślusarz <marcin.slusarz@...il.com>
Cc:	David Rientjes <rientjes@...gle.com>, Tejun Heo <tj@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: cpu_hotplug vs oom_notify_list: possible circular locking
 dependency detected

On Sun, Jul 19, 2015 at 11:58:18AM +0200, Marcin Ślusarz wrote:
> 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@...ux.vnet.ibm.com>
> > > > > 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@...il.com>
> > > > >     Reported-by: David Rientjes <rientjes@...gle.com>
> > > > >     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > > 
> > > > Acked-by: David Rientjes <rientjes@...gle.com>
> > > 
> > > 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@...il.com>

Thank you, applied!

							Thanx, Paul

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