RE: PATCH: Fix race in cpu_down (hotplug cpu)

Li, Shaohua
2005-09-19T04:49:17Z

Hi,
>
>On Mon, Sep 19, 2005 at 01:28:38PM +1000, Nigel Cunningham wrote:
>> There is a race condition in taking down a cpu
(kernel/cpu.c::cpu_down).
>> A cpu can already be idling when we clear its online flag, and we do
not
>> force the idle task to reschedule. This results in __cpu_die timing
out.
>
>"when we clear its online flag" - This happens in take_cpu_down in the
>context of stopmachine thread. take_cpu_down also ensures that idle
>thread runs when it returns (sched_idle_next). So when idle thread
runs,
>it should notice that it is offline and invoke play_dead. So I don't
>understand why __cpu_die should time out.
I guess Nigel's point is cpu_idle is preempted before take_cpu_down. If
the preempt occurs after the cpu_is_offline check, when the cpu (after
sched_idle_next) goes into idle again, nobody can wake it up. Nigel,
isn't it?

Thanks,
Shaohua