cpu_down: move migrate_enable() back

Commit 08c1ab68, "hotplug-use-migrate-disable.patch", intends to
use migrate_enable()/migrate_disable() to replace that combination
of preempt_enable() and preempt_disable(), but actually in
!CONFIG_PREEMPT_RT_FULL case, migrate_enable()/migrate_disable()
are still equal to preempt_enable()/preempt_disable(). So that
followed cpu_hotplug_begin()/cpu_unplug_begin(cpu) would go schedule()
to trigger schedule_debug() like this:

_cpu_down()
	|
	+ migrate_disable() = preempt_disable()
	|
	+ cpu_hotplug_begin() or cpu_unplug_begin()
		|
		+ schedule()
			|
			+ __schedule()
				|
				+ preempt_disable();
				|
				+ __schedule_bug() is true!

So we should move migrate_enable() as the original scheme.

Cc: stable-rt@vger.kernel.org
Signed-off-by: Tiejun Chen <tiejun.chen@windriver.com>

diff --git a/kernel/cpu.c b/kernel/cpu.c
index c96cb6735d35e4d40e70ff778847f6a1f9d44a17..ddc57402539fc58246066bc2f49cc2ef23ea1afe 100644 (file)
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -665,6 +665,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
                err = -EBUSY;
                goto restore_cpus;
        }
+       migrate_enable();
 
        cpu_hotplug_begin();
        err = cpu_unplug_begin(cpu);
@@ -737,7 +738,6 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
 out_release:
        cpu_unplug_done(cpu);
 out_cancel:
-       migrate_enable();
        cpu_hotplug_done();
        if (!err)
                cpu_notify_nofail(CPU_POST_DEAD | mod, hcpu);