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? Maintainers are going to
need to understand the implications of the problem before the
sta...@kernel.org annotation is warranted.
> 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();
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.
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