Re: [PATCH 2/5] stop_machine: reimplement using cpu_stop

[Andrew Morton <akpm@linux-foundation.org>]

On Thu,  6 May 2010 18:52:49 +0200
Tejun Heo <tj@kernel.org> wrote:

> Reimplement stop_machine using cpu_stop.  As cpu stoppers are
> guaranteed to be available for all online cpus,
> stop_machine_create/destroy() are no longer necessary and removed.
> 
> With resource management and synchronization handled by cpu_stop, the
> new implementation is much simpler.  Asking the cpu_stop to execute
> the stop_cpu() state machine on all online cpus with cpu hotplug
> disabled is enough.
> 
> stop_machine itself doesn't need to manage any global resources
> anymore, so all per-instance information is rolled into struct
> stop_machine_data and the mutex and all static data variables are
> removed.
> 
> The previous implementation created and destroyed RT workqueues as
> necessary which made stop_machine() calls highly expensive on very
> large machines.  According to Dimitri Sivanich, preventing the dynamic
> creation/destruction makes booting faster more than twice on very
> large machines.  cpu_stop resources are preallocated for all online
> cpus and should have the same effect.
> 
> ...
>
> @@ -361,9 +357,6 @@ int disable_nonboot_cpus(void)
>  {
>  	int cpu, first_cpu, error;
>  
> -	error = stop_machine_create();
> -	if (error)
> -		return error;
>  	cpu_maps_update_begin();
>  	first_cpu = cpumask_first(cpu_online_mask);
>  	/*

kernel/cpu.c: In function 'disable_nonboot_cpus':
kernel/cpu.c:397: warning: 'error' may be used uninitialized in this function

Looks like this is a real bug if the cpumask happens to contain only a
single CPU for some reason.

--- a/kernel/cpu.c~a
+++ a/kernel/cpu.c
@@ -355,7 +355,8 @@ static cpumask_var_t frozen_cpus;
 
 int disable_nonboot_cpus(void)
 {
-	int cpu, first_cpu, error;
+	int cpu, first_cpu;
+	int error = 0;
 
 	cpu_maps_update_begin();
 	first_cpu = cpumask_first(cpu_online_mask);
_