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    SubjectRe: [PATCH RFC] stop_machine: make stop_machine safe and efficient to call early
    On Fri, 30 Sep 2011 10:00:06 -0700
    Jeremy Fitzhardinge <jeremy@goop.org> wrote:

    > On 09/30/2011 09:34 AM, Steven Rostedt wrote:
    > > Doesn't interrupts need to be disabled here too? As stop machine
    > > functions also guarantee that they will not be interrupted by
    > > interrupts. -- Steve
    >
    > Good point.
    >
    > J
    >
    > Make stop_machine() safe to call early in boot, before SMP has been
    > set up, by simply calling the callback function directly if there's
    > only one CPU online.

    Being a trusting soul, I shall assume that you presently have or soon
    will have some code which requires this change?

    >
    > ...
    >
    > --- a/kernel/stop_machine.c
    > +++ b/kernel/stop_machine.c
    > @@ -485,6 +485,17 @@ int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
    > .num_threads = num_online_cpus(),
    > .active_cpus = cpus };
    >
    > + if (smdata.num_threads == 1) {
    > + unsigned long flags;
    > + int ret;
    > +
    > + local_save_flags(flags);
    > + ret = (*fn)(data);
    > + local_irq_restore(flags);
    > +
    > + return ret;
    > + }

    It is quite unobvious to readers why this code exists. Therefore, we
    should tell them.

    Also, bug. local_save_flags() is used to read the irq flags - it does
    not actually disable interrupts. We want local_irq_save() here. None
    of these interfaces are documented and their naming system is crappy.
    Cause, effect.


    --- a/kernel/stop_machine.c~stop_machine-make-stop_machine-safe-and-efficient-to-call-early-fix
    +++ a/kernel/stop_machine.c
    @@ -486,10 +486,14 @@ int __stop_machine(int (*fn)(void *), vo
    .active_cpus = cpus };

    if (smdata.num_threads == 1) {
    + /*
    + * Handle the case where stop_machine() is called early in boot
    + * before SMP startup.
    + */
    unsigned long flags;
    int ret;

    - local_save_flags(flags);
    + local_irq_save(flags);
    ret = (*fn)(data);
    local_irq_restore(flags);

    _



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