Re: [PATCH v2 2/2] Do not change worker's running cpu in cmci_rediscover().

[Tang Chen <tangchen@cn.fujitsu.com>]

On 10/20/2012 01:21 AM, Luck, Tony wrote:
>> In this case, the following BUG_ON in try_to_wake_up_local() will be triggered:
>> BUG_ON(rq != this_rq());
>
> Logically this looks OK - what is the test case to trigger this?  I've done a moderate
> amount of testing of cpu online/offline while injecting corrected errors (when testing
> the CMCI storm patches) ... but didn't see this problem.

Hi Tony, Borislav,

Here is my case.

I have 2 nodes, node0 and node1. node1 could be hotpluged.
node0 has cpu0 ~ cpu15, node1 has cpu16 ~ cpu31.

I online all the cpus on node1, and hot-remove node1 directly.

When this problem is triggered, current is a work thread.

For example: cpu20 is dying. current is on cpu21, it migrates
itself to cpu22.

Assume current is process1, and it is a work thread.

        cpu21                        cpu22
p1:
....
cmci_rediscover()
|-set_cpus_allowed_ptr()
   |-stop_one_cpu()
     |-create a work to excute migration_cpu_stop()
       |-wait_for_completion()
         |-wait_for_common()
           |-might_sleep()
Here, p1 gives up cpu21.

The work starts:
migration_cpu_stop()
|-migrate p1 to cpu22

                                On cpu22, p1 wakes up:
                                p1:
                                In wait_for_common()
                                   |-do_wait_for_common()
                                     |-schedule_timeout()
                                       |-schedule()
                                         |-__schedule()
                                           |-try_to_wake_up_local()
                                             |-wq_worker_sleeping()
                                             |-BUG_ON(rq != this_rq())

On cpu22, wq_worker_sleeping() uses p1's worker_pool to find a worker
to go on to execute p1. But p1's worker_pool is on cpu21, and p1 is now
on cpu22. So the BUG_ON(rq != this_rq()) is triggered.

Thanks. :)

>
> -Tony
>