On 18.07.2017 16:51, Stanislav Kinsburskiy wrote:
> From: Srivatsa S. Bhat <srivatsa.b...@linux.vnet.ibm.com>
>
> Backport of upstream commit: e30a293e8ad7e6048d6d88bcc114094f964bd67b
>
> https://jira.sw.ru/browse/PSBM-68620
>
> Fixes race between CPU add/del and per-cpu notifier add/del.
>
> 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.
Just for clearness.
The original description confuses, as it's possible to think we're fixing
deadlock here.
The patch is about: "We iterate over online cpu list, but we have not
prohibited its
modifications before. Fix that".
> Cc: Li RongQing <roy.qing...@gmail.com>
> Cc: Sasha Levin <sasha.le...@oracle.com>
> Cc: Andrew Morton <a...@linux-foundation.org>
> Cc: Chris Metcalf <cmetc...@tilera.com>
> Cc: Ingo Molnar <mi...@kernel.org>
> Acked-by: David S. Miller <da...@davemloft.net>
> Signed-off-by: Srivatsa S. Bhat <srivatsa.b...@linux.vnet.ibm.com>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wyso...@intel.com>
> Signed-off-by: Stanislav Kinsburskiy <skinsbur...@virtuozzo.com>
Reviewed-by: Kirill Tkhai <ktk...@virtuozzo.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;
>
>
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