Re: [PATCH 3/8] Use process freezer for cpu-hotplug

[Nathan Lynch <ntl@pobox.com>]

Gautham R Shenoy wrote:
> This patch implements process_freezer based cpu-hotplug
> core. 
> The sailent features are:
> o No more (un)lock_cpu_hotplug.
> o No more CPU_LOCK_ACQUIRE and CPU_LOCK_RELEASE. Hence no per-subsystem
>   hotcpu mutexes.
> o Calls freeze_process/thaw_processes at the beginning/end of
>   the hotplug operation.

...

> @@ -133,7 +111,11 @@ static int _cpu_down(unsigned int cpu)
>  	if (!cpu_online(cpu))
>  		return -EINVAL;
>  
> -	raw_notifier_call_chain(&cpu_chain, CPU_LOCK_ACQUIRE, hcpu);
> +	if (freeze_processes(FE_HOTPLUG_CPU)) {
> +		thaw_processes(FE_HOTPLUG_CPU);
> +		return -EBUSY;
> +	}
> +

If I'm understanding correctly, this will cause

# echo 0 > /sys/devices/system/cpu/cpuX/online

to sometimes fail, and userspace is expected to try again?  This will
break existing applications.

Perhaps drivers/base/cpu.c:store_online should retry as long as
cpu_up/down return -EBUSY.  That would avoid a userspace-visible
interface change.