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

[Nigel Cunningham <ncunningham@cyclades.com>]

Hi.

On Mon, 2005-09-19 at 14:48, Li, Shaohua wrote:
> 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?

Maybe I'm just an ignoramus, but I was thinking (without being a
scheduler expert at all) that if the idle thread was already running,
trying to set it up to run next might possibly have zero effect. I've
added a bit of debugging code to try and see in better detail what's
happening.

Regards,

Nigel

> Thanks,
> Shaohua
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