[Devel] [PATCH RHEL7 COMMIT] ms/net/core/flow.c: Fix CPU hotplug callback registration

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-514.26.1.vz7.33.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-514.26.1.vz7.33.10
------>
commit 2d7d7eb08268527613f2b3d54b9c388159305199
Author: Srivatsa S. Bhat <srivatsa.bhat at linux.vnet.ibm.com>
Date:   Tue Jul 18 18:20:27 2017 +0400

    ms/net/core/flow.c: Fix CPU hotplug callback registration
    
    Backport of upstream commit: e30a293e8ad7e6048d6d88bcc114094f964bd67b
    
    https://jira.sw.ru/browse/PSBM-68620
    
    Fixes race between CPU add/del and per-cpu notifier add/del.
    
    Signed-off-by: Stanislav Kinsburskiy <skinsbursky at virtuozzo.com>
    
    Below is original commit message:
    
    =========================================================================
    
    Subsystems that want to register CPU hotplug callbacks, as well as perform
    initialization for the CPUs that are already online, often do it as shown
    below:
    
    	get_online_cpus();
    
    	for_each_online_cpu(cpu)
    		init_cpu(cpu);
    
    	register_cpu_notifier(&foobar_cpu_notifier);
    
    	put_online_cpus();
    
    This is wrong, since it is prone to ABBA deadlocks involving the
    cpu_add_remove_lock and the cpu_hotplug.lock (when running concurrently
    with CPU hotplug operations).
    
    Instead, the correct and race-free way of performing the callback
    registration is:
    
    	cpu_notifier_register_begin();
    
    	for_each_online_cpu(cpu)
    		init_cpu(cpu);
    
    	/* Note the use of the double underscored version of the API */
    	__register_cpu_notifier(&foobar_cpu_notifier);
    
    	cpu_notifier_register_done();
    
    Fix the code in net/core/flow.c by using this latter form of callback
    registration.
    
    Cc: Li RongQing <roy.qing.li at gmail.com>
    Cc: Sasha Levin <sasha.levin at oracle.com>
    Cc: Andrew Morton <akpm at linux-foundation.org>
    Cc: Chris Metcalf <cmetcalf at tilera.com>
    Cc: Ingo Molnar <mingo at kernel.org>
    Acked-by: David S. Miller <davem at davemloft.net>
    Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat at linux.vnet.ibm.com>
    Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki at intel.com>
---
 net/core/flow.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/core/flow.c b/net/core/flow.c
index e2664a2..20b3ac3 100644
--- a/net/core/flow.c
+++ b/net/core/flow.c
@@ -450,6 +450,8 @@ int flow_cache_init(struct net *net)
 	if (!fc->percpu)
 		return -ENOMEM;
 
+	cpu_notifier_register_begin();
+
 	for_each_online_cpu(i) {
 		if (flow_cache_cpu_prepare(fc, i))
 			goto err;
@@ -457,7 +459,9 @@ int flow_cache_init(struct net *net)
 	fc->hotcpu_notifier = (struct notifier_block){
 		.notifier_call = flow_cache_cpu,
 	};
-	register_hotcpu_notifier(&fc->hotcpu_notifier);
+	__register_hotcpu_notifier(&fc->hotcpu_notifier);
+
+	cpu_notifier_register_done();
 
 	setup_timer(&fc->rnd_timer, flow_cache_new_hashrnd,
 		    (unsigned long) fc);
@@ -473,6 +477,8 @@ int flow_cache_init(struct net *net)
 		fcp->hash_table = NULL;
 	}
 
+	cpu_notifier_register_done();
+
 	free_percpu(fc->percpu);
 	fc->percpu = NULL;