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

[Steven Rostedt <rostedt@goodmis.org>]

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?

> 
>   - 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.

-- Steve