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    SubjectRe: [PATCH 2/4] stop_machine: reimplement using cpuhog
    On Mon, Mar 08, 2010 at 07:27:08PM +0100, Oleg Nesterov wrote:
    > On 03/08, Heiko Carstens wrote:
    > >
    > > On Tue, Mar 09, 2010 at 12:53:21AM +0900, Tejun Heo wrote:
    > > >
    > > > diff --git a/arch/s390/kernel/time.c b/arch/s390/kernel/time.c
    > > > index 65065ac..afe429e 100644
    > > > --- a/arch/s390/kernel/time.c
    > > > +++ b/arch/s390/kernel/time.c
    > > > @@ -397,7 +397,6 @@ static void __init time_init_wq(void)
    > > > if (time_sync_wq)
    > > > return;
    > > > time_sync_wq = create_singlethread_workqueue("timesync");
    > > > - stop_machine_create();
    > > > }
    > > >
    > > > /*
    > >
    > > The reason we introduced stop_machine_create/destroy was to have a non-failing
    > > variant that doesn't rely on I/O.
    > > If we ever see a timesync machine check no I/O will succeed (it blocks) until
    > > clocks have been synchronized. That means also that we rely on the non-blocking
    > > semantics that those functions must have that are called via stop_machine.
    > > This isn't true anymore with the cpu hog infrastructure:
    > > if passed a blocking function that could wait on I/O we won't see any progress
    > > anymore and the machine is dead.
    >
    > Could you please spell?
    >
    > How cpuhog can make a difference? Afaics, we shouldn't pass a blocking callback
    > to hog_cpus/hog_one_cpu.

    Well, it might me true that this shouldn't be done. But I don't see a reason why
    in general it wouldn't work to pass a function that would block.
    So it's just a matter of time until somebody uses it for such a purpose.
    For the current stop_machine implementation it would be broken to pass a blocking
    function (preemption disabled, interrupts disabled).


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