linux-kernel - [PATCH tip/core/rcu 16/16] rcu: Don't disable CPU hotplug during OOM notifiers

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Date:	Fri, 17 Jul 2015 15:20:16 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	bobby.prani@...il.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 16/16] rcu: Don't disable CPU hotplug during OOM notifiers

From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>

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>
---
 kernel/rcu/tree_plugin.h | 2 --
 1 file changed, 2 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 9e922f111d63..80a7c17907fe 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1679,12 +1679,10 @@ static int rcu_oom_notify(struct notifier_block *self,
 	 */
 	atomic_set(&oom_callback_count, 1);
 
-	get_online_cpus();
 	for_each_online_cpu(cpu) {
 		smp_call_function_single(cpu, rcu_oom_notify_cpu, NULL, 1);
 		cond_resched_rcu_qs();
 	}
-	put_online_cpus();
 
 	/* Unconditionally decrement: no need to wake ourselves up. */
 	atomic_dec(&oom_callback_count);
-- 
1.8.1.5

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