Re: [patch 19/20] ACPI/processor: Use cpu_hotplug_disable() instead of get_online_cpus()

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Saturday, April 15, 2017 07:01:26 PM Thomas Gleixner wrote:
> Converting the hotplug locking, i.e. get_online_cpus(), to a percpu rwsem
> unearthed a circular lock dependency which was hidden from lockdep due to
> the lockdep annotation of get_online_cpus() which prevents lockdep from
> creating full dependency chains.
> 
> CPU0                    CPU1
> ----                    ----
> lock((&wfc.work));
>                          lock(cpu_hotplug_lock.rw_sem);
>                          lock((&wfc.work));
> lock(cpu_hotplug_lock.rw_sem);
> 
> This dependency is established via acpi_processor_start() which calls into
> the work queue code. And the work queue code establishes the reverse
> dependency.
> 
> This is not a problem of get_online_cpus() recursion, it's a possible
> deadlock undetected by lockdep so far.
> 
> The cure is to use cpu_hotplug_disable() instead of get_online_cpus() to
> protect the probing from acpi_processor_start().
> 
> There is a side effect to this: cpu_hotplug_disable() makes a concurrent
> cpu hotplug attempt via the sysfs interfaces fail with -EBUSY, but that
> probing usually happens during the boot process where no interaction is
> possible. Any later invocations are infrequent enough and concurrent
> hotplug attempts are so unlikely that the danger of user space visible
> regressions is very close to zero. Anyway, thats preferrable over a real
> deadlock.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
> Cc: Len Brown <lenb@kernel.org>
> Cc: linux-acpi@vger.kernel.org

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

> ---
>  drivers/acpi/processor_driver.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> --- a/drivers/acpi/processor_driver.c
> +++ b/drivers/acpi/processor_driver.c
> @@ -268,9 +268,9 @@ static int acpi_processor_start(struct d
>  		return -ENODEV;
>  
>  	/* Protect against concurrent CPU hotplug operations */
> -	get_online_cpus();
> +	cpu_hotplug_disable();
>  	ret = __acpi_processor_start(device);
> -	put_online_cpus();
> +	cpu_hotplug_enable();
>  	return ret;
>  }
>  
> 
>