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SubjectRe: [PATCH]CPU hotplug breaks wake_up_new_task
On Tue, May 31, 2005 at 12:35:09AM -0700, Shaohua Li wrote:
>
> Hi,
> There is a race condition at wake_up_new_task at CPU hotplug case.
> Say do_fork
> copy_process (which sets new forked task's current cpu,
> cpu_allowed)
> <-------- the new forked task's current cpu is offline
> wake_up_new_task
> wake_up_new_task will put the forked task into a dead cpu.
>
> Thanks,
> Shaohua
>
> Signed-off-by: Shaohua Li<shaohua.li@intel.com>
> ---
>
> linux-2.6.11-rc5-mm1-root/kernel/sched.c | 25
.... Deleted....
> /*
> * wake_up_new_task - wake up a newly created task for the first
> time.
> *
> @@ -1426,9 +1430,20 @@ void fastcall wake_up_new_task(task_t *
> runqueue_t *rq, *this_rq;
>
> rq = task_rq_lock(p, &flags);
> - BUG_ON(p->state != TASK_RUNNING);
> this_cpu = smp_processor_id();
> cpu = task_cpu(p);
> +#ifdef CONFIG_HOTPLUG_CPU
> + while (!cpu_online(cpu)) {
> + cpu = task_select_online_cpu(cpu, p);
> + set_task_cpu(p, cpu);
> + task_rq_unlock(rq, &flags);
> + /* CPU hotplug might occur here */
> + rq = task_rq_lock(p, &flags);
> + this_cpu = smp_processor_id();
> + cpu = task_cpu(p);
> + }
> +#endif

The while() loop doesnt look pretty here.. could you try to
disable preempt, and see the problem goes away? or use
get_cpu()/put_cpu() combo when you get this_cpu?

Just wondering if the code would be a little more simpler in this
case.

Nick should have something to say ...

Cheers,
ashok
-
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