Re: [PATCH] intel_rapl: Correct hotplug correction

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 05/22/2014 02:53 PM, Borislav Petkov wrote:
> From: Borislav Petkov <bp@suse.de>
> 
> So 009f225ef050 ("powercap, intel-rapl: Fix CPU hotplug callback
> registration") says how get_/put_online_cpus() should be replaced with
> this cpu_notifier_register_begin/_done().
> 
> But they're still there so what's up?
> 

Ok, so I retained that because the comments in the code said that
the caller of rapl_cleanup_data() should hold the hotplug lock.

Here is the snippet from the patch's changelog:

    ...
    Fix the intel-rapl code in the powercap driver by using this latter form
    of callback registration. But retain the calls to get/put_online_cpus(),
    since they also protect the function rapl_cleanup_data(). By nesting
    get/put_online_cpus() *inside* cpu_notifier_register_begin/done(), we avoid
    the ABBA deadlock possibility mentioned above.

But looking closer at the code, I think the only requirement is that
rapl_cleanup_data() should be protected against CPU hotplug, and we
don't actually need to hold the cpu_hotplug.lock per-se.

cpu_notifier_register_begin()/end() also provide equivalent protection
against CPU hotplug. So we should be able to remove the get/put_online_cpus()
from intel-rapl driver.

Jacob/Srinivas, is the above assumption correct for rapl?

Regards,
Srivatsa S. Bhat

> Let me do what was supposed to be done.
> 
> Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
> Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Signed-off-by: Borislav Petkov <bp@suse.de>
> ---
>  drivers/powercap/intel_rapl.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/drivers/powercap/intel_rapl.c b/drivers/powercap/intel_rapl.c
> index d9a0770b6c73..9055f3df1f64 100644
> --- a/drivers/powercap/intel_rapl.c
> +++ b/drivers/powercap/intel_rapl.c
> @@ -1377,8 +1377,6 @@ static int __init rapl_init(void)
> 
>  	cpu_notifier_register_begin();
> 
> -	/* prevent CPU hotplug during detection */
> -	get_online_cpus();
>  	ret = rapl_detect_topology();
>  	if (ret)
>  		goto done;
> @@ -1390,7 +1388,6 @@ static int __init rapl_init(void)
>  	}
>  	__register_hotcpu_notifier(&rapl_cpu_notifier);
>  done:
> -	put_online_cpus();
>  	cpu_notifier_register_done();
> 
>  	return ret;
>