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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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