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    SubjectRe: + generic-ipi-fix-the-race-between-generic_smp_call_function_-and-hotplug_cfd.patch added to -mm tree
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    On Sun, 2009-09-20 at 19:55 -0700, Xiao Guangrong wrote:
    >
    > Suresh Siddha wrote:
    > > On Wed, 2009-09-16 at 20:00 -0700, Xiao Guangrong wrote:
    > >> How about manual check/handle pending IPI interruption in the CPU context?
    > >> like this:
    > >> --- a/kernel/cpu.c
    > >> +++ b/kernel/cpu.c
    > >> @@ -173,6 +173,9 @@ static int __ref take_cpu_down(void *_param)
    > >> struct take_cpu_down_param *param = _param;
    > >> int err;
    > >>
    > >> + generic_smp_call_function_interrupt();
    > >> + generic_smp_call_function_single_interrupt();
    > >> +
    > >
    > > At this place, how will you ensure that the smp_call_function initiated
    > > by this dying cpu has reached and got serviced at its destination?
    > >
    >
    > Suresh, sorry for my poor English, Do you mean that how we ensure it has
    > pending IPI request in the dying cpu?
    >
    > generic_smp_call_function_*() will check it, if the cpu has pending request,
    > then handle it, else directly return.
    >

    I am referring to the missing csd_lock_wait() here that you had in the
    first version of your patch. Let's say, if cpu X is going offline, we
    need to ensure that the smp_call_function() initiated by cpu X (i.e.,
    smp_call_function IPI sent to some other cpu's from cpu X) got serviced
    before cpu X goes offline. We can't do csd_lock_wait() here, as that
    might deadlock (as all the other cpu's are already in stop machine with
    interrupts disabled).

    > > All the other cpu's have disabled interrupts in the stop machine state
    > > by the time we come here and we can't wait.
    > >
    >
    > Why we can't wait? It manual check/handle the pending IPI request not wait
    > interruption happen.
    >
    > It not has race here because all cpu's interruption is disabled, and it not
    > make stop machine slow because only the dying cpu can enter take_cpu_down(),
    > we just wait the dying cpu handle it's pending request.
    >
    > Am I misunderstand something?
    >
    > Thanks,
    > Xiao
    >



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