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>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <20091216170517.500272612@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 291ac586f37ff93e047391be46b3db90d5f80445..1c8ddd6ee9402fdf2f6853c6142f3a8d0cb57da6 100644 (file)
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -209,6 +209,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
                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)