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

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Date:	Tue, 14 Jul 2015 16:48:24 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Marcin Ślusarz <marcin.slusarz@...il.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 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>