Re: 3.0.23-rt39 BUG: scheduling while atomic, _cpu_down()

[Frank Rowand <frank.rowand@am.sony.com>]

On 03/09/12 18:03, Steven Rostedt wrote:
> On Fri, 2012-03-09 at 17:51 -0800, Frank Rowand wrote:
>> Hi Thomas,
>> (and Steve, since you've been mucking about in the hotplug code,)
>>
>> Yet another bit of fallout related to hotplug.
>>
>> This appears to affect 3.0-rt1, 3.0.23-rt29, and 3.2.9-rt17.
>> (Thus probably all 3.x versions, but I haven't been masochistic
>> enough to check.)
>>
>> If not defined CONFIG_PREEMPT_RT_FULL then
> 
> So you're saying that there's a bug with PREEMPT_RT off?

Yes, that is the problem.  But it is due to the RT patches,
not present in mainline.


>>   - hotplug_lock() is a mutex_lock()
>>   - cpu_hotplug_begin() calls hotplug_lock()
>>   - _cpu_down() calls cpu_hotplug_begin() with preemption
>>     disabled (disabled by the migrate_disable())
>>
>> The call to cpu_hotplug_begin() is not in a preempt disabled
>> region until hotplug-use-migrate-disable.patch.  This patch
>> moves the preempt_enable() (aka migrate_enable()) from just
>> before calling cpu_hotplug_begin() to after "out_cancel:".
>>
>> The BUG can be triggered by:
>>
>>    echo 0 > /sys/devices/system/cpu/cpu1/online 
> 
> Note, there's a lot more wrong with hotplug in rt than this. The hotplug
> code really needs a total rewrite for RT to work with it. It probably
> should have a total rewrite in mainline anyway.

Yep, I've been watching all the hotplug related conversations with interest...

-Frank