Re: [Next] CPU Hotplug test failures on powerpc

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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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