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    SubjectRe: [PATCH] ACPI/CPUIDLE: prevent setting pm_idle to NULL
    On Mon, 28 Jul 2008, Andi Kleen wrote:
    > > + /*
    > > + * Fall back to the default idle loop, when pm_idle_save had
    > > + * been initialized.
    > > + */
    > > + if (pm_idle_save) {
    > > + pm_idle = pm_idle_save;
    > > + /* Relies on interrupts forcing exit from idle. */
    > > + synchronize_sched();
    > > + }
    >
    > I think it would be better to fall back to default_idle (which
    > might need to be exported) when the old pointer is NULL.

    No, falling back to default_idle is wrong as hell. When pm_idle was
    set to mwait_idle or whatever then you override pm_idle to
    default_idle for no good reason.

    The problem here is that the acpi/cpuidle code can be in a state where
    the _save/old variables _ARE_ NULL because they had not been
    initialized with the original pm_idle before the module is removed or
    the cst state changes. So all we have to do is to prevent pm_idle to
    be set to NULL.

    > Now with your patch
    > the cpuidle idle code would run with inconsistent state for some time,
    > which is probably not good.

    Err, this happens when the original pm_idle pointer _is_ restored. But
    if the _save/old pointer is NULL we crash the system and that's what
    my patch prevents.

    So nothing runs with an inconsistent state. If pm_idle_save contains
    the original pm_idle value we restore, if it is NULL we do not touch
    pm_idle and keep the original value.

    Thanks,

    tglx


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