lkml.org 
[lkml]   [2010]   [Apr]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 2/2] cpuhotplug: make get_online_cpus() scalability by using percpu counter
On 04/05, Lai Jiangshan wrote:
>
> 1) get_online_cpus() must be allowed to be called recursively, so I added
> get_online_cpus_nest for every task for new code.

Well, iirc one of the goals of

cpu-hotplug: replace lock_cpu_hotplug() with get_online_cpus()
86ef5c9a8edd78e6bf92879f32329d89b2d55b5a

was avoiding the new members in task_struct. I leave this up to you
and Gautham.


Lai, I didn't read this patch carefully yet (and I can't apply it to
Linus's tree). But at first glance,

> void put_online_cpus(void)
> {
> ...
> + if (!--current->get_online_cpus_nest) {
> + preempt_disable();
> + __get_cpu_var(refcount)--;
> + if (cpu_hotplug_task)
> + wake_up_process(cpu_hotplug_task);

This looks unsafe. In theory nothing protects cpu_hotplug_task from
exiting if refcount_sum() becomes zero, this means wake_up_process()
can hit the freed/reused/unmapped task_struct. Probably cpu_hotplug_done()
needs another synhronize_sched() before return.

OTOH, I do not understand why the result of __get_cpu_var(refcount)
must be visible to refcount_sum() if we race with cpu_hotplug_begin(),
so it seems to me cpu_hotplug_begin() also needs synchronize_sched()
before refcount_sum().

Oleg.



\
 
 \ /
  Last update: 2010-04-05 19:07    [W:0.056 / U:0.544 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site