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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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