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    Subject[PATCH 02/12] sched: Fix set_cpu_active() in cpu_down()
    Sachin found cpu hotplug test failures on powerpc, which made the
    kernel hang on his POWER box.

    The problem is that we fail to re-activate a cpu when a hot-unplug
    fails. Fix this by moving the de-activation into _cpu_down after doing
    the initial checks.

    Remove the synchronize_sched() calls and rely on those implied by
    rebuilding the sched domains using the new mask.

    Reported-by: Sachin Sant <sachinp@in.ibm.com>
    Signed-off-by: Xiaotian Feng <dfeng@redhat.com>
    Tested-by: Sachin Sant <sachinp@in.ibm.com>
    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    ---
    kernel/cpu.c | 24 +++---------------------
    1 file changed, 3 insertions(+), 21 deletions(-)

    Index: linux-2.6/kernel/cpu.c
    ===================================================================
    --- linux-2.6.orig/kernel/cpu.c
    +++ linux-2.6/kernel/cpu.c
    @@ -209,6 +209,7 @@ static int __ref _cpu_down(unsigned int
    return -ENOMEM;

    cpu_hotplug_begin();
    + set_cpu_active(cpu, false);
    err = __raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE | mod,
    hcpu, -1, &nr_calls);
    if (err == NOTIFY_BAD) {
    @@ -280,18 +281,6 @@ int __ref cpu_down(unsigned int cpu)
    goto out;
    }

    - set_cpu_active(cpu, false);
    -
    - /*
    - * Make sure the all cpus did the reschedule and are not
    - * using stale version of the cpu_active_mask.
    - * This is not strictly necessary becuase stop_machine()
    - * that we run down the line already provides the required
    - * synchronization. But it's really a side effect and we do not
    - * want to depend on the innards of the stop_machine here.
    - */
    - synchronize_sched();
    -
    err = _cpu_down(cpu, 0);

    out:
    @@ -382,19 +371,12 @@ int disable_nonboot_cpus(void)
    return error;
    cpu_maps_update_begin();
    first_cpu = cpumask_first(cpu_online_mask);
    - /* We take down all of the non-boot CPUs in one shot to avoid races
    + /*
    + * We take down all of the non-boot CPUs in one shot to avoid races
    * with the userspace trying to use the CPU hotplug at the same time
    */
    cpumask_clear(frozen_cpus);

    - for_each_online_cpu(cpu) {
    - if (cpu == first_cpu)
    - continue;
    - set_cpu_active(cpu, false);
    - }
    -
    - synchronize_sched();
    -
    printk("Disabling non-boot CPUs ...\n");
    for_each_online_cpu(cpu) {
    if (cpu == first_cpu)
    --



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