Xiaotian Feng wrote:
Could follow be possible? We know there's cpu 0 and cpu 1,
offline cpu1 > done
offline cpu0 > false
consider this in cpu_down code,
int __ref cpu_down(unsigned int cpu)
{
<snip>
set_cpu_active(cpu, false); // here, we set cpu 0 to inactive
synchronize_sched();
err = _cpu_down(cpu, 0);
out:
<snip>
}
Then in _cpu_down code:
static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
{
<snip>
if (num_online_cpus() == 1) // if we're trying to
offline cpu0, num_online_cpus will be 1
return -EBUSY; // after return back
to cpu_down, we didn't change cpu 0 back to active
if (!cpu_online(cpu))
return -EINVAL;
if (!alloc_cpumask_var(&old_allowed, GFP_KERNEL))
return -ENOMEM;
<snip>
}
Then cpu 0 is not active, but online, then we try to offline cpu1, .......
This can not be exposed because x86 does not have
/sys/devices/system/cpu0/online.
I guess following patch fixes this bug.
Just tested this one on the POWER box and the test passed.
I did not observe the hang.
Thanks
-Sachin
---
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 291ac58..21ddace 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -199,14 +199,18 @@ static int __ref _cpu_down(unsigned int cpu, int
tasks_frozen)
.hcpu = hcpu,
};
- if (num_online_cpus() == 1)
+ if (num_online_cpus() == 1) {
+ set_cpu_active(cpu, true);
return -EBUSY;
+ }
if (!cpu_online(cpu))
return -EINVAL;
- if (!alloc_cpumask_var(&old_allowed, GFP_KERNEL))
+ if (!alloc_cpumask_var(&old_allowed, GFP_KERNEL)) {
+ set_cpu_active(cpu, true);
return -ENOMEM;
+ }
cpu_hotplug_begin();
err = __raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE | mod,
Unless of course, I messed up, which appears to be rather likely given
these problems ;-)
--
---------------------------------
Sachin Sant
IBM Linux Technology Center
India Systems and Technology Labs
Bangalore, India
---------------------------------
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