On 08/02/2014 05:54 AM, David Rientjes wrote:
> On Thu, 31 Jul 2014, Lai Jiangshan wrote:
>
>> If the smpboot_register_percpu_thread() is called after
>> smpboot_create_threads()
>> but before __cpu_up(), the smpboot thread of the online-ing CPU is not
>> created,
>> and it results a bug. So we use get_online_cpus() to prevent it.
>>
>
> Do you have an example of the bug to include?
Sorry, no, I don't have.
> Maintainers are going to
> need to understand the implications of the problem before the
> sta...@kernel.org annotation is warranted.
It is possible that smpboot_register_percpu_thread() can be called
any time in current kernel. Repeating the module ehca and check while
repeating online/offline the CPUs, the bug is possible to hit. I have not such
devices to test.
Let Thomas make the choice.
>
>> smpboot_unregister_percpu_thread() travels all possible CPU, it doesn't need
>> get_online_cpus() which is removed in the patch.
>>
>> CC: Thomas Gleixner <t...@linutronix.de>
>> Cc: Rusty Russell <ru...@rustcorp.com.au>
>> Cc: Peter Zijlstra <pet...@infradead.org>
>> Cc: Srivatsa S. Bhat <srivatsa.b...@linux.vnet.ibm.com>
>> CC: sta...@kernel.org
>> Signed-off-by: Lai Jiangshan <la...@cn.fujitsu.com>
>> ---
>> kernel/smpboot.c | 4 ++--
>> 1 files changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/smpboot.c b/kernel/smpboot.c
>> index eb89e18..8adab87 100644
>> --- a/kernel/smpboot.c
>> +++ b/kernel/smpboot.c
>> @@ -279,6 +279,7 @@ int smpboot_register_percpu_thread(struct
>> smp_hotplug_thread *plug_thread)
>> unsigned int cpu;
>> int ret = 0;
>>
>> + get_online_cpus();
>> mutex_lock(&smpboot_threads_lock);
>> for_each_online_cpu(cpu) {
>> ret = __smpboot_create_thread(plug_thread, cpu);
>> @@ -291,6 +292,7 @@ int smpboot_register_percpu_thread(struct
>> smp_hotplug_thread *plug_thread)
>> list_add(&plug_thread->list, &hotplug_threads);
>> out:
>> mutex_unlock(&smpboot_threads_lock);
>> + put_online_cpus();
>> return ret;
>> }
>
> I think the {get,put}_online_cpus() pair should be nested inside the
> smpboot_threads_lock for better lock ordering since not all cases
> smpboot_threads_lock will require it.
>
> That way, you can also do put_online_cpus() before
> smpboot_destroy_threads(), which you have already proven doesn't need it:
>
> @@ -280,14 +280,17 @@ int smpboot_register_percpu_thread(struct
> smp_hotplug_thread *plug_thread)
> int ret = 0;
>
> mutex_lock(&smpboot_threads_lock);
> + get_online_cpus();
get_online_cpus() can't be nested in smpboot_threads_lock.
> for_each_online_cpu(cpu) {
> ret = __smpboot_create_thread(plug_thread, cpu);
> if (ret) {
> + put_online_cpus();
> smpboot_destroy_threads(plug_thread);
> goto out;
> }
> smpboot_unpark_thread(plug_thread, cpu);
> }
> + put_online_cpus();
> list_add(&plug_thread->list, &hotplug_threads);
> out:
> mutex_unlock(&smpboot_threads_lock);
>
>> EXPORT_SYMBOL_GPL(smpboot_register_percpu_thread);
>> @@ -303,11 +305,9 @@ EXPORT_SYMBOL_GPL(smpboot_register_percpu_thread);
>> */
>> void smpboot_unregister_percpu_thread(struct smp_hotplug_thread
>> *plug_thread)
>> {
>> - get_online_cpus();
>> mutex_lock(&smpboot_threads_lock);
>> list_del(&plug_thread->list);
>> smpboot_destroy_threads(plug_thread);
>> mutex_unlock(&smpboot_threads_lock);
>> - put_online_cpus();
>> }
>> EXPORT_SYMBOL_GPL(smpboot_unregister_percpu_thread);
>
> This makes sense.
> .
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
