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SubjectRe: PATCH: Fix race in cpu_down (hotplug cpu)
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On Mon, 2005-09-19 at 11:53 +0530, Srivatsa Vaddagiri wrote:
> On Mon, Sep 19, 2005 at 04:11:11PM +1000, Nigel Cunningham wrote:
> > > Ok, that makes sense. Nigel, could you confirm which idle routine you are
> > > using?
> >
> > >From dmesg:
> >
> > monitor/mwait feature present.
> > using mwait in idle threads.
>
> Ok, that may explain why __cpu_die is timing out for you! Can you try a
> (untested, I am afraid) patch on these lines:
>
> --- process.c.org 2005-09-19 11:44:57.000000000 +0530
> +++ process.c 2005-09-19 11:48:28.000000000 +0530
> @@ -245,16 +245,18 @@ EXPORT_SYMBOL_GPL(cpu_idle_wait);
> */
> static void mwait_idle(void)
> {
> + int cpu = raw_smp_processor_id();
> +
> local_irq_enable();
>
> if (!need_resched()) {
> set_thread_flag(TIF_POLLING_NRFLAG);
> do {
> __monitor((void *)&current_thread_info()->flags, 0, 0);
> - if (need_resched())
> + if (need_resched() || cpu_is_offline(cpu))
> break;
if the breakpoint is here, you will still have trouble.

> __mwait(0, 0);
> - } while (!need_resched());
> + } while (!need_resched() || !cpu_is_offline(cpu));
> clear_thread_flag(TIF_POLLING_NRFLAG);
> }
> }
>
>
> Other idle routines will need similar fix.
>
> > Ok, but what about default_idle?
>
> default_idle should be fine as it is. IOW it should not cause __cpu_die to
> timeout.
Why default_idle should be fine? it can be preempted before the
'local_irq_disable' check. Even with Nigel's patch, there is a very
small window at safe_halt (after 'sti' but before 'hlt').

Thanks,
Shaohua

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